Remove beta flag from incremental logging

What does this MR do?

Self-managed customers are aiming to remove NFS from their scaled-out deployments of GitLab, including:

• I'm working on a customer's ticket (for GitLab team members: zd, SF) scoping how to get their Rails servers off NFS.
• #24177 (comment 467574940)

Two locations depend on enabling the feature flag for incremental logging:

• /var/opt/gitlab/gitlab-rails/shared/artifacts/
• /var/opt/gitlab/gitlab-ci/builds/

This MR is a proposal to remove the warnings that this is a beta feature. This is a blocker for the customer I'm working with to remove NFS, and on their behalf, I am seeking to establish if this is production-ready yet, or not.

There are a number of discussions already about whether this is production ready / generally available (GA). including:

• the epic that was a requirement for GitLab.com and further work is tracked in epic: &3791
• the issue for removing the feature flag

This feature is now live on GitLab.com, &4275 (closed) was a pre-requisite to remove the above mentioned NFS mount points from GitLab.com servers, and it's enabled by default for customers running the helm deployment of GitLab.

If there are specific risks or limitations that would affect self-managed customers, it will affect customers running the helm deployment, so we should identify what they are.

While the documentation being updated is grouprunner this feature flag is actually ~"group::continuous integration".

Related issues

• &4275 (closed)
• &3791

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