gh-112826: Add a "What's New" Entry About _thread._is_main_interpreter (gh-112853)

As of gh-112661, the threading module expects the _thread module to have a _is_main_interpreter(), which is used in the internal threading._shutdown().  This change causes a problem for anyone that replaces the _thread module with a custom one (only if they don't provide _is_main_interpreter()).  They need to be sure to add it for 3.13+, thus this PR is adding a note in "What's New".

This also forward-ports the "What's New" entry from 3.12 (gh-112850).  Note that we do not also forward-port the fix in that PR.  The fix is there only due to a regression from 3.12.0. There is no regression in 3.13+.