Removes database migrations up to 16-1-stable-ee

What does this MR do and why?

Remove database migrations up to 20230615202511 – until 16-1-stable-ee.

Follow up from !139940 (merged)

Changes in this MR:

• Delete db/migrate and db/post_migrate introduced in 16-1-stable-ee up to 20230615202511
• 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 16.2 - 20230616082958

NOTE 1

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

NOTE 2

The following specs were removed:

Spec	Reason
spec/lib/gitlab/background_migration/backfill_snippet_repositories_spec.rb	Removed because constraint "check_0dd5948e38" is now in init_structure.sql
spec/lib/gitlab/background_migration/migrate_human_user_type_spec.rb	Removed because constraint "check_0dd5948e38" is now in init_structure.sql
spec/lib/gitlab/background_migration/backfill_admin_mode_scope_for_personal_access_tokens_spec.rb	Removed because constraint "check_b8d60815eb" is now in init_structure.sql
spec/lib/gitlab/background_migration/cleanup_personal_access_tokens_with_nil_expires_at_spec.rb	constraint "check_b8d60815eb" is now in init_structure.sql
spec/lib/gitlab/background_migration/mark_duplicate_npm_packages_for_destruction_spec.rb	fails because of idx_packages_on_project_id_name_version_unique_when_npm violation

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 16-1-stable-ee
• Deleted all migrations up to 16-1-stable-ee version
• Copied the structure.sql file to init_structure.sql (from 16-1-stable-ee version)

How to set up and validate locally

Generate a diff file from master branch

1. Checkout master branch

git checkout master

2. Re-create the database

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

3. Migration to version 20230615202511 (last migration from 15-9-stable-ee)

bin/rails db:migrate:main VERSION=20230615202511

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-16-1 branch

1. Checkout current branch

git checkout 409574-remove-old-migrations-to-16-1

2. Re-create the database

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

3. Migrate to version 20211202041233 (only the init_schema migration)

bin/rails db:migrate:main VERSION=20211202041233

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

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

5. Reset the changes

git reset --hard 409574-remove-old-migrations-to-16-1

Check the changes between files

git diff 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 (closed)