Add feature flag for disabling reCaptcha

What does this MR do?

Disabling reCaptcha

That is the main purpose of this MR. It disables recaptcha in case a feature flag is set to true.

Disabling recaptcha means that issues and merge requests identified as spam by Akismet are still stored as normal issues. This does not apply to snippets as they are the biggest source of span.

Other improvements

expect/allow_any_instance_of was changed to expect_next_instance in files touched by the change.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

#64023 (moved)