Reset severity_levels default

What does this MR do?

Reset severity_levels default to the previous values in both model and existing records.

As the default was not set on the DB level there shouldn't be many approval rules matching this criteria in production.

Related issue: #335333 (closed)

Before migration

gitlabhq_development=# select id, severity_levels from approval_project_rules where array_length(severity_levels, 1) = 6;
 id |             severity_levels
----+-----------------------------------------
 53 | {critical,high,medium,low,unknown,info}
(1 row)

After migration

gitlabhq_development=# select id, severity_levels from approval_project_rules where array_length(severity_levels, 1) = 6;
 id | severity_levels
----+-----------------
(0 rows)

gitlabhq_development=# select id, severity_levels from approval_project_rules where id=53;
 id |     severity_levels
----+-------------------------
 53 | {unknown,high,critical}
(1 row)

Migration

$ bundle exec rails db:migrate
WARNING: This version of GitLab depends on gitlab-shell 13.19.1, but you're running 13.19.0. Please update gitlab-shell.
== 20210819183128 ResetSeverityLevelsToNewDefault: migrating ==================
-- execute("update approval_project_rules set severity_levels='{unknown, high, critical}' where array_length(severity_levels, 1) = 6;")
   -> 0.0057s
== 20210819183128 ResetSeverityLevelsToNewDefault: migrated (0.0058s) =========

Rollback(no-op)

$ bundle exec rails db:rollback
WARNING: This version of GitLab depends on gitlab-shell 13.19.1, but you're running 13.19.0. Please update gitlab-shell.
== 20210819183128 ResetSeverityLevelsToNewDefault: reverting ==================
== 20210819183128 ResetSeverityLevelsToNewDefault: reverted (0.0000s) =========

Database-lab

explain update approval_project_rules set severity_levels='{unknown, high, critical}' where array_length(severity_levels, 1) = 6;
Session: 6114
Plan with execution:
 ModifyTable on public.approval_project_rules  (cost=0.00..13266.84 rows=1150 width=199) (actual time=1708.489..1708.491 rows=0 loops=1)
   Buffers: shared hit=90959 read=3501 dirtied=1802 written=202
   I/O Timings: read=1402.915 write=0.000
   ->  Seq Scan on public.approval_project_rules  (cost=0.00..13266.84 rows=1150 width=199) (actual time=36.529..201.219 rows=5592 loops=1)
   
[...SKIP...]

Full execution plan (The text in the preview above has been cut)
Other artifacts are provided in the thread. Recommendations:
❗ SeqScan is used – Consider adding an index Show details
❗ Query processes too much data to return a relatively small number of rows. – Reduce data cardinality as early as possible during the execution, using one or several of the following techniques: new indexes, partitioning, query rewriting, denormalization. See the visualization of the plan to understand which plan nodes are the main bottlenecks. Show details
Summary:

Time: 1.710 s
  - planning: 1.018 ms
  - execution: 1.709 s
    - I/O read: 1.403 s
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 90959 (~710.60 MiB) from the buffer pool
  - reads: 3501 (~27.40 MiB) from the OS file cache, including disk I/O
  - dirtied: 1802 (~14.10 MiB)
  - writes: 202 (~1.60 MiB)

Details and visualization: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/6114/commands/20654. (edited)

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

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• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

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• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

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