Consider removal of APPLICATION_JSON_UTF8 (and MediaTypes)

[rwinch]

MediaTypes was put in as a temporary measure for getting updated to Spring Framework 7 so that we could more carefully consider the removal of APPLICATION_JSON_UTF8.

Spring Framework removed APPLICATION_JSON_UTF8 because:

Deprecated as of 5.2 in favor ofAPPLICATION_JSON since major browsers like Chrome now comply with the specification and interpret correctly UTF-8 special characters without requiring a UTF-8 parameter.

We should review if APPLICATION_JSON_UTF8 can be removed from Spring Security. In order to do this, we need to review Spring Security implementations that use it and see if the corresponding specifications require APPLICATION_JSON_UTF8 or if APPLICATION_JSON is acceptable. In particular, this appears to impact a number of OAuth classes, so their corresponding specs must be reviewed.

[rwinch]

Closing in favor of gh-17070