Service Template deprecation UI updates

Problem to Solve

After adding Project Integration Management features in %13.3 and a deprecation note about Service Templates in %13.4, we expected to see a dramatic reduction in usage. However, in the run-up to fully deprecating this feature in %14.0, we discovered that usage of this feature is still growing slowly.

Proposal

Plan per @leipert (#325196 (comment 532528192)):

1. frontend: Create a more visible User Callout for Admin Users of Instances with Service Templates configured.
   • This user callout should link to this feedback issue.
   • This likely needs some backend support to correctly identify the condition
   • @aqualls / @lvanc: We need a good copy / UX where to display this one.
2. frontend: Update the existing callout on the settings page to match the contents of the one created in 1..
3. frontend (#326962 (closed)) / backend: Move Service Template code to be behind a Feature Flag that is enabled per default.

%14.0: Disable feature flag per default.

🖍 Clickable prototype

/cc @lvanc @mnohr