Update status checks docs and remove incorrect text in settings

What does this MR do and why?

This MR does the following:

1. Enables the only_allow_merge_if_all_status_checks_passed feature flag by default, so that self-hosted customers will get the feature.

2. Updates the docs to reflect the feature flag default on change, and also added steps on how to enable blocking the MR merge when external status checks fail.

3. Fixes a sentence on the project settings -> Merge requests -> Status checks section that's no longer correct with the feature flag enabled:

Before	After

How to set up and validate locally

1. Go to any project's settings -> Merge requests.
2. Verify that the "Failures do not block merges" sentence does not show under the Status checks section.
3. Disable the only_allow_merge_if_all_status_checks_passed feature flag.
4. Verify that the "Failures do not block merges" sentence does show after disabling the feature flag.

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 #372340 (closed)