Add maintainer_note field to CI runners table

What does this MR do and why?

Describe in detail what your merge request does and why.

This is a simple MR to add a maintainer_note field to the ci_runners table in accordance to #348299 (closed).

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

In a local GDK console, do:

> Ci::Runner.first.update!(maintainer_note: 'Test admin notes')
  Ci::Runner Load (2.0ms)  SELECT "ci_runners".* FROM "ci_runners" ORDER BY "ci_runners"."id" ASC LIMIT 1 /*application:console,db_config_name:main,line:(pry):60:in `__pry__'*/
  TRANSACTION (0.2ms)  BEGIN /*application:console,db_config_name:main,line:/lib/gitlab/database/schema_cache_with_renamed_table.rb:25:in `columns'*/
  Feature::FlipperFeature Load (5.5ms)  SELECT "features".* FROM "features" /*application:console,db_config_name:main,line:/lib/feature.rb:49:in `persisted_names'*/
  ActsAsTaggableOn::Tagging Load (6.9ms)  SELECT "taggings".* FROM "taggings" WHERE "taggings"."taggable_id" = 4 AND "taggings"."taggable_type" = 'Ci::Runner' /*application:console,db_config_name:main,line:/app/models/ci/runner.rb:391:in `tag_list'*/
  ActsAsTaggableOn::Tag Load (2.0ms)  SELECT "tags".* FROM "tags" INNER JOIN "taggings" ON "tags"."id" = "taggings"."tag_id" WHERE "taggings"."taggable_id" = 4 AND "taggings"."taggable_type" = 'Ci::Runner' AND (taggings.context = 'tags' AND taggings.tagger_id IS NULL) /*application:console,db_config_name:main,line:/app/models/ci/runner.rb:391:in `tag_list'*/
  Ci::RunnerProject Exists? (5.1ms)  SELECT 1 AS one FROM "ci_runner_projects" WHERE "ci_runner_projects"."runner_id" = 4 LIMIT 1 /*application:console,db_config_name:main,line:/app/models/ci/runner.rb:515:in `no_projects'*/
  Ci::RunnerNamespace Exists? (2.4ms)  SELECT 1 AS one FROM "ci_runner_namespaces" WHERE "ci_runner_namespaces"."runner_id" = 4 LIMIT 1 /*application:console,db_config_name:main,line:/app/models/ci/runner.rb:521:in `no_groups'*/
  Ci::Runner Update (0.8ms)  UPDATE "ci_runners" SET "updated_at" = '2022-01-07 16:22:41.829191', "maintainer_note" = 'Test admin notes' WHERE "ci_runners"."id" = 4 /*application:console,db_config_name:main,line:(pry):60:in `__pry__'*/
  ActsAsTaggableOn::Tag Load (1.0ms)  SELECT "tags".* FROM "tags" WHERE (name = 'shell' OR name = 'gdk' OR name = 'mac' OR name = 'test') /*application:console,db_config_name:main,line:(pry):60:in `__pry__'*/
  ActsAsTaggableOn::Tag Load (0.5ms)  SELECT "tags".* FROM "tags" INNER JOIN "taggings" ON "tags"."id" = "taggings"."tag_id" WHERE "taggings"."taggable_id" = 4 AND "taggings"."taggable_type" = 'Ci::Runner' AND (taggings.context = 'tags' AND taggings.tagger_id IS NULL) /*application:console,db_config_name:main,line:(pry):60:in `__pry__'*/
  ActsAsTaggableOn::Tag Load (0.3ms)  SELECT "tags".* FROM "tags" WHERE (name = 'shell' OR name = 'gdk' OR name = 'mac' OR name = 'test') /*application:console,db_config_name:main,line:(pry):60:in `__pry__'*/
  ActsAsTaggableOn::Tag Load (0.4ms)  SELECT "tags".* FROM "tags" INNER JOIN "taggings" ON "tags"."id" = "taggings"."tag_id" WHERE "taggings"."taggable_id" = 4 AND "taggings"."taggable_type" = 'Ci::Runner' AND (taggings.context = 'tags' AND taggings.tagger_id IS NULL) /*application:console,db_config_name:main,line:(pry):60:in `__pry__'*/
  TRANSACTION (0.4ms)  COMMIT /*application:console,db_config_name:main,line:/lib/gitlab/database.rb:298:in `commit'*/
=> true

> Ci::Runner.first.maintainer_note
  Ci::Runner Load (1.5ms)  SELECT "ci_runners".* FROM "ci_runners" ORDER BY "ci_runners"."id" ASC LIMIT 1 /*application:console,db_config_name:main,line:(pry):61:in `__pry__'*/
=> "Test admin notes"

> Ci::Runner.first.update!(maintainer_note: '1'*256)
ActiveRecord::RecordInvalid: Validation failed: Maintainer note is too long (maximum is 255 characters)

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.

Part of #348299 (closed)