Ai Gateway - remove legacy audience

Problem to solve

In gitlab-org/gitlab#426572 (closed) we introduced support for the new universal JWT token.

We changed the token audience to gitlab-ai-gateway. The model gateway still maintains compatibility with the legacy gitlab-code-suggestions audience

After we complete:

• CDot: put universal token into SeatLink as CS t... (gitlab-org/gitlab#426573 - closed)
• Gitlab Rails: Consolidate SaaS mechanism for is... (gitlab-org/gitlab#426950 - closed)

Proposal

We can remove legacy gitlab-code-suggestions audience from Ai Gateway. Already issued tokens have expiration date for 3 days, so we shuold be fine removing this in next milestone once gitlab-org/gitlab#426573 (closed) and gitlab-org/gitlab#426950 (closed) are completed

Further details

Links / references