Captcha Keypad 2.0.0

Drupal 12 support, and the first release on the semantic versioning branch.
Drupal 8 and 9 are no longer supported: core_version_requirement is now
^10.2 || ^11 || ^12.

Fixes a captcha bypass in standalone mode, where the expected code was sent to
the browser in a hidden field and validation compared that field against the
submitted response. It is now signed with the site private key.

Fixes the keypad being unusable in a browser on Drupal 10 and later, and a
wrong code producing an error with no message.

Issue #3596099.