Remove beta mark from APIs introduced for JEP-202

[jglick]

@oleg-nenashev pointed out in jenkinsci/jep#280 that for the JEP to be considered final we would expect these APIs to become unrestricted. They have long been in use in plugin releases.

Proposed changelog entries

• Pluggable Artifact Storage: Make the VirtualFile API generally available to plugin developers.
  • JEP-202: https://github.com/jenkinsci/jep/tree/master/jep/202

Proposed upgrade guidelines

N/A

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least 2 approvals for the pull request and no outstanding requests for change
• Conversations in the pull request are over OR it is explicit that a reviewer does not block the change
• Changelog entries in the PR title and/or Proposed changelog entries are correct
• Proper changelog labels are set so that the changelog can be generated automatically
• If the change needs additional upgrade steps from users, upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the PR title. (example)
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[oleg-nenashev]

Fine with me. We have 2+ reference implementations, including one created by external developers (Azure artifact storage)

[oleg-nenashev]

We may merge it in 24 hours if there is no negative feedback. Please see the merge process documentation for more information about the merge process

[oleg-nenashev]

restarted CI. Will merge once/if it passes