Describe all wiring changes from v0.50 to v0.51 (main) in simapp/CHANGELOG.md

[julienrbrt]

We recently created a simapp/CHANGELOG.md that is complementary to the UPGRADING.md.
While the UGPRADING.md describes the important changes in the app wiring and the modules, as well as new or changes of notable concepts, simapp/CHANGELOG.md should be updated to contain an exhaustive list of changes in simapp.
A user can then easily read what has changed in simapp and in the module wiring.

--

• Check diff from v0.50.x to main git difftool main..v0.50.6 -- simapp/
• For each wiring change describe it briefly in simapp/CHANGELOG.md and link the PR that introduce the change
• Verify there is no duplication in the root CHANGELOG.md (and if so, and totally duplicated -- no other change), remove the entry

[julienrbrt]

Completed in #21310