PKCE is now always on (remove FF_ENABLE_PKCE)

What does this MR do?

Remove FF_ENABLE_PKCE; it's now always on.

We preserve mixed-version tolerance in the proxy step (i.e. we accept an /auth request without a code_challenge); this keeps upgrades safe for SM installs who didn't enable the FF, which is likely to be almost all installs.

See Implement PKCE under an FF (!1328 - merged).

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines