[DOCU-2296] Plugins: Remove deprecated X-credential-username, single-…

…source (#4318)

* removed x-credential-username in favor of x-credential-identifier; moved upstream headers info into an include

* conditional for upstream headers include

* single-source hmac auth; minor clean-up; bring back an overwritten feature update

* single-source basic auth

* single-source mtls-auth

* single-source key-auth-enc; minor tweak to mtls auth

* single-source key-auth

* remove conditionals from changelogs

* single-source ldap-auth plugin

* single-source oauth2-introspection plugin

* vale edits