Change `IS_GITLAB_EE` to `FOSS_ONLY`

What does this MR do?

This removes the usage of IS_GITLAB_EE and prefer to use FOSS_ONLY that has the following states:

1. nil/empty string: use CE or EE depending on sources
2. false: use CE or EE depending on sources
3. true: use CE always

This follows the discussion in: #33875 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines

Closes: #33875 (closed)