Removes database migrations up to 16-2-stable-ee

What does this MR do and why?

Remove database migrations up to 20230717200940 – until 16-2-stable-ee.

Follow up from !139940 (merged)

Changes in this MR:

• Delete db/migrate and db/post_migrate introduced in 16-2-stable-ee up to 20230717200940
• Delete related specs
• Move 16-1-stable-ee db/structure.sql changes to init_structure.sql
• db:rollback job was updated with the first migration from 16.3 - 20230718020825

NOTE 1

We decided not to rename the 20211202041233_init_schema.rb file, to avoid problems with migrations

How did I perform the squash:

I used the squash script: bundle exec rake gitlab:db:squash\[16-1-stable-ee\]:

• Compared the diff between master and 16-2-stable-ee
• Deleted all migrations up to 16-2-stable-ee version
• Copied the structure.sql file to init_structure.sql (from 16-2-stable-ee version)

How to set up and validate locally

Generate a diff file from master branch

1. Checkout master branch (revision 14b8b0416a0f27be331ae34b151b2702f4c80723) — This revision makes sure that the same migrations are present in both branches, master and 409574-remove-old-migrations-to-16-2

git checkout 14b8b0416a0f27be331ae34b151b2702f4c80723

2. Re-create the database

gdk stop && gdk start db && bin/rails db:drop db:create

3. Migrate the database

bin/rails db:migrate:main

4. Store structure.sql the diff in a temp file

cat db/structure.sql > tmp/master.diff

5. Reset the changes

git reset --hard origin

Generate a diff file from 409574-remove-old-migrations-to-16-2 branch

1. Checkout current branch

git checkout 409574-remove-old-migrations-to-16-2

2. Re-create the database

gdk stop && gdk start db && bin/rails db:drop db:create

3. Migrate the database

bin/rails db:migrate:main

4. Store structure.sql the diff in a temp file

cat db/structure.sql > tmp/squash.diff

5. Reset the changes

git reset --hard origin/409574-remove-old-migrations-to-16-2

Check the changes between files

 git diff --no-index tmp/master.diff tmp/squash.diff

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #409574