Remove obsolete ignore_column directives

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR removes in a first commit the obsolete column :semver from ci_runners. I also ran bundle exec rake db:obsolete_ignored_columns and removed the remaining obsolete ignore_column directives (some of which were ready to be removed since %14.2). I didn't remove all the obsolete ones, as unfortunately some seem to be still in use, so I'll leave those for the original author.

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Before

$ bundle exec rake db:obsolete_ignored_columns
The following `ignored_columns` are obsolete and can be removed:
ApplicationSetting:
 - max_package_files_for_package_destruction Remove after 2022-03-22 with 14.9
 - user_email_lookup_limit        Remove after 2022-04-18 with 15.0
 - enforce_ssh_key_expiration     Remove after 2022-07-22 with 15.2
 - enforce_pat_expiration         Remove after 2022-07-22 with 15.2
Ci::Runner:
 - semver                         Remove after 2022-08-22 with 15.4
Clusters::Applications::Ingress:
 - modsecurity_enabled            Remove after 2021-07-22 with 14.2
 - modsecurity_mode               Remove after 2021-07-22 with 14.2
Project:
 - container_registry_enabled     Remove after 2021-09-22 with 14.4
Vulnerabilities::Finding:
 - migrated_to_new_structure      Remove after 2022-08-22 with 15.4

WARNING: Removing columns is tricky because running GitLab processes may still be using the columns.

See also https://docs.gitlab.com/ee/development/avoiding_downtime_in_migrations.html#dropping-columns

After

$ bundle exec rake db:obsolete_ignored_columns
The following `ignored_columns` are obsolete and can be removed:
ApplicationSetting:
 - user_email_lookup_limit        Remove after 2022-04-18 with 15.0
Project:
 - container_registry_enabled     Remove after 2021-09-22 with 14.4

How to set up and validate locally

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