[release/6.0-staging] Sys.RT.Caching.MemoryCache - Remove unhandled exception handler

[StephenMolloy]

• Description - Timer was updated in 5.0 to improve performance, but is prone to deadlocks when manipulated in an unhandled exception handler. Which MemoryCache does. And MemoryCache is probably more likely to witness unhandled exceptions like OOM than typical code.
• Customer Impact - The deadlock was affecting a first-party team (running services on Substrate) from migrating to .Net 6 and 8.
• Regression? - Yes, since .Net 5.0. And also a regression from NetFx.
• Risk Low.

Original Issue: #64115 (and #102666)
.Net 9.0 PR: #105853
(This was originally fixed in 9.0 with #103937, but after discussion with servicing shiproom, we have decided on a different approach. These servicing PR's are minimal and do not include the removal of the AppDomain.DomainUnload handler, per this comment.)

System.Runtime.Caching is an OOB package that ships alongside the runtime.

[carlossanlop]

@StephenMolloy Friendly reminder that Code Complete for the October Release is September 9. If you want this fix to be included in that release, please merge this PR before that date. Otherwise, it will have to wait until November.