Eliminate `acts_as_taggable` gem for project topics

What does this MR do?

This MR is the second of two steps to resolve #335946 (closed). In the first step (see !67574 (merged)), the project topics data was migrated to two separate tables and it was ensured, that the project topics work correctly with old and new data during the background migration.

In this MR, the use of the acts_as_taggable gem for project topics is finally eliminated. In addition, the temporary index tmp_index_taggings_on_id_where_taggable_type_project is removed again.

Note: Since !67574 (merged) (with background migration) was merged in %14.3, this MR will apparently have to wait for %14.4.

🛠 with ❤ at Siemens

/cc @bufferoverflow

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