(dev/core#3166) "Metadata" cache - Strictly separate by version

(dev/core#3166) "Metadata" cache - Strictly separate by version

This addresses some symptoms identified in the discussion of 3166.

Before
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The "metadata" cache is always stored with the name `metadata`.

If you load a newer codebase, it still reads `metadata` from the older code base (until something explicitly clears the cache).

After
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The "metadata" cache is always stored with the name `metadata_{version}`, eg `metadata_5_48_0`.

If you load a newer codebase (eg `5.49.0`), it will ignore the older `metadata_5_48_0` and create a new `metadata_5_49_0`.

Comments
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The upgrade system also clears cache, but many users rely on navigating through the site to get to the upgrader.

The change could make sense with other caches, but I only applied the change to the `metadata` cache because there is _some_ risk;
eg if some caller bypasses the cache objects. (I was also slightly concerned about the possibility the some caller relies
on `$cache->getName()` or `$cache->getPrefix()` or similar... but I haven't been able to find any methods like that...). It seems
safer to put that sort of change into a monthly update rather than minor patch.