Remove ci_stop_populating_p_ci_build_tags feature flag

What does this MR do and why?

Removes the ci_stop_populating_p_ci_build_tags feature flag which controlled whether build tags are written to the p_ci_build_tags table.

With the feature flag enabled (now the default), build tags are stored in ci_job_definitions instead of p_ci_build_tags. This MR finalizes that migration by:

1. Removing the feature flag and its YAML definition
2. Removing BuildsTagsConfiguration - since uses_taggings? returned false, BulkInsert#insert! would return early without doing any work for builds. The class is no longer needed.
3. Skipping the save_tags callback in Ci::Build - uses skip_callback :save, :after, :save_tags to prevent the inherited callback from Ci::Taggable from running, since tags are now stored in job definitions
4. Removing Ci::BulkInsertableTags prepend from Ci::Build - this module's purpose was to suppress individual save_tags calls when bulk inserting. Since the save_tags callback is now skipped for builds, the prepend is unnecessary. The module itself remains for Ci::Runner which still uses bulk tag insertion.
5. Removing with_bulk_insert_tags wrapper from pipeline creation - no longer needed since the save_tags callback is skipped for builds
6. Removing the explicit pipeline.transaction block - was only needed for atomicity between pipeline.save! and bulk_insert_tags!
7. Removing the unused taggable_statuses method
8. Updating pending_builds factory - uses find_or_create_all_with_like_by_name instead of named to ensure tags exist when creating pending builds in tests

Changelog: removed

References

• Feature issue: #580301 (closed)
• Rollout issue: #583505 (closed)
• Introduction MR: !215235 (merged)

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