Enable Ref advertisement caching by default on mirrors

Problem Definition

Mirrors can receive many concurrent ref advertisement requests (mainly by CI/CD tools).
These requests will lead to scm tickets being consumed very quickly and operations to being queued.
In some extreme cases, this could even lead to operations not being performed at all when the timeouts are hit.

Suggested Solution

Enable the "Ref advertisement caching" by default on mirrors

Workaround

In the interim, enable the ref advertisement cache by adding the following to the bitbucket.properties file for each mirror:

plugin.bitbucket-scm-cache.refs.enabled=true

Note
no plugins can be installed on mirrors so the refs will always be in the expected status in the caches because. There is no risk that a plugin will manipulate the refs without the caching plugin detecting these changes.

Note 2
It is not possible to set or check the status of the ref cache by using REST API endpoints on the mirror.