Fix foreign_key_exists? migration helper

What does this MR do and why?

Use .column_exists? rather than checking ActiveRecord::Migrator.current_version, as backported migration break this assumption.

See #394760 (comment 1303224068).

Screenshots or screen recordings

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How to set up and validate locally

1. export GITLAB_HOME=/srv/gitlab

2. Start pre 15.7 version, for example 15.6.8.

docker run \
  --hostname gitlab.example.com \
  --publish 3080:80 \
  --name gitlab \
  --restart always \
  --volume $GITLAB_HOME/config:/etc/gitlab \
  --volume $GITLAB_HOME/logs:/var/log/gitlab \
  --volume $GITLAB_HOME/data:/var/opt/gitlab \
  --shm-size 256m \
gitlab/gitlab-ee:15.6.8-ee.0

3. Follow the docs to upgrade to gitlab/gitlab-ee:15.9.2-ee.0, migrations will fail as reported with PG::UndefinedColumn: ERROR: column postgres_foreign_keys.constrained_table_name does not exist

4. Build a new image that includes the fix from this MR

Dockerfile:

FROM gitlab/gitlab-ee:15.9.2-ee.0

COPY migration_helpers.patch /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/migration_helpers.patch
RUN cd /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database && patch < migration_helpers.patch

migration_helpers.patch:

--- a/lib/gitlab/database/migration_helpers.rb
+++ b/lib/gitlab/database/migration_helpers.rb
@@ -367,12 +367,12 @@ def validate_foreign_key(source, column, name: nil)
 
       def foreign_key_exists?(source, target = nil, **options)
         # This if block is necessary because foreign_key_exists? is called in down migrations that may execute before
-        # the postgres_foreign_keys view had necessary columns added, or even before the view existed.
+        # the postgres_foreign_keys view had necessary columns added.
         # In that case, we revert to the previous behavior of this method.
         # The behavior in the if block has a bug: it always returns false if the fk being checked has multiple columns.
         # This can be removed after init_schema.rb passes 20221122210711_add_columns_to_postgres_foreign_keys.rb
         # Tracking issue: https://gitlab.com/gitlab-org/gitlab/-/issues/386796
-        if ActiveRecord::Migrator.current_version < 20221122210711
+        unless connection.column_exists?('postgres_foreign_keys', 'constrained_table_name')
           return foreign_keys(source).any? do |foreign_key|
             tables_match?(target.to_s, foreign_key.to_table.to_s) &&
                 options_match?(foreign_key.options, options)

5. Build with docker build -t gitlab-fk-error:latest ..

6. Start the container

docker run \
  --hostname gitlab.example.com \
  --publish 3080:80 \
  --name gitlab \
  --restart always \
  --volume $GITLAB_HOME/config:/etc/gitlab \
  --volume $GITLAB_HOME/logs:/var/log/gitlab \
  --volume $GITLAB_HOME/data:/var/opt/gitlab \
  --shm-size 256m \
 gitlab-fk-error:latest 

7. The migrations will execute successfully.

MR acceptance checklist

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