Switch ActsOnTaggable model's DB connection to ci database

What does this MR do and why?

Switch ActsOnTaggable model's DB connection to ci database

We need to do two things:

1. Override the LB connection (which is based off the base class's LB)
2. Override the connection_specification_name for other Rails connection methods that LB does not handle such as retrieve_connection

The specs from https://gitlab.com/gitlab-org/gitlab/-/blob/876909a20993d054a88c254b74a24a28e23042dd/spec/models/ci/runner_spec.rb#L1032 should cover this, in that it ensures that a SQL query joining from ci_runners to tags runs correctly.

Related issue: #343747 (closed)

Screenshots or screen recordings

Before:

[1] pry(main)> ActsAsTaggableOn::Tag.connection.execute('select current_database()').first
   (0.4ms)  select current_database() /*application:console,db_config_name:main,line:(pry):1:in `__pry__'*/
=> {"current_database"=>"gitlabhq_development"}
[2] pry(main)> ActsAsTaggableOn::Tag.retrieve_connection.execute('select current_database()').first
   (0.9ms)  select current_database() /*application:console,db_config_name:main,line:(pry):2:in `__pry__'*/
=> {"current_database"=>"gitlabhq_development"}

After, with ci: database in config/database.yml:

[1] pry(main)> ActsAsTaggableOn::Tag.connection.execute('select current_database()').first
   (0.4ms)  select current_database() /*application:console,db_config_name:ci,line:(pry):1:in `__pry__'*/
=> {"current_database"=>"gitlabhq_development_ci"}
[2] pry(main)> ActsAsTaggableOn::Tag.retrieve_connection.execute('select current_database()').first
   (1.0ms)  select current_database() /*application:console,db_config_name:ci,line:(pry):2:in `__pry__'*/
=> {"current_database"=>"gitlabhq_development_ci"}

How to set up and validate locally

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

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