Rendering: Don't cache RTE value conversion (closes #20867)

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes #20867

Description

With the removal of snapshot cache, the Snapshot property cache level is treated as if it was an Element level cache (see comment here).

This has worked seemingly fine, because `Element? level caching was effectively not happening. But with the fix in #20681, the linked issue popped up quite immediately.

I have been back and forth on this one a few times, and the only immediate (stable) solution is to set the RTE value conversion cache level to None instead of Snapshot. I have also added a task to our internal backlog to investigate this further.

At first glance, the None cache level looks like quite the performance degradation. In effect, though, it really isn't:

• Snapshot in V14 and below meant a re-rendering for each request; the cache lasted for the duration of the snapshot, which was effectively per request.
• Snapshot in V15+ should have been behaving like Element, but due to the above-mentioned error, all Element level caching was effectively discarded.

Testing this PR

See the linked issue for details.

I have verified that this works both for RTEs and Block Editors (including nested blocks).

[AndyButland]

Have verified that the original issue reported is resolved by this change.