Write doc describing what to do when updating Major version/TFM

[wtgodbe]

There are lots of subtle fixes/workarounds that need to be made/updated when updating branding to a new Major version & updating TFM. We should write a doc as we do this for .Net 7 so future engineers won't be lost.

Relevant (Branding/ingesting first 7.0-branded changes from runtime & EFCore): #35402

[wtgodbe]

@dougbu I assigned this to both of us, whoever winds up doing the TFM work can pick up the doc too.