Enable scan_result_any_merge_request by default

What does this MR do and why?

This MR enables scan_result_any_merge_request feature flag by default. The feature was rolled out globally 1 week ago.

It also enabled scan_result_policies_block_force_push which has been rolled out globally.

It also updates documentation and removes mentions of previously removed security_policies_branch_exceptions feature flag.

Please note that the MR is in draft because based on this discussion, the feature should be released together with [Feature flag] Rollout of `scan_result_policies... (#427260 - closed), which is in verification as of now. I updated this MR to include also scan_result_policies_block_force_push, which has been verified and rolled out globally as well.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #423988 (closed) and #427260 (closed)