Update ChibiOS, ChibiOS-Contrib. (qmk#13056)

* Update ChibiOS, ChibiOS-Contrib.

* Add instructions.

* Wrong remote name

* Explicit version tag.

* Add link to procedure on the breaking changes page.

docs/breaking_changes.md

@@ -94,7 +94,6 @@ This happens immediately after the previous `develop` branch is merged.
 * GitHub Actions
     * [ ] Create a PR for `develop`
     * [ ] Make sure travis comes back clean
-    * [ ] **Turn off 'Automatically delete head branches' for the repository** -- confirm with @qmk/directors that it is done before continuing
 * `qmk_firmware` git commands
     * [ ] `git checkout master`
     * [ ] `git pull --ff-only`

docs/chibios_upgrade_instructions.md

@@ -0,0 +1,56 @@
+# ChibiOS Upgrade Procedure
+
+ChibiOS and ChibiOS-Contrib need to be updated in tandem -- the latter has a branch tied to the ChibiOS version in use and should not be mixed with different versions.
+
+## Getting ChibiOS
+
+* `svn` Initialisation:
+    * Only needed to be done once
+    * You might need to separately install `git-svn` package in your OS's package manager
+    * `git svn init --stdlayout --prefix='svn/' http://svn.osdn.net/svnroot/chibios/`
+    * `git remote add qmk [email protected]:qmk/ChibiOS.git`
+* Updating:
+    * `git svn fetch`
+    * First time around this will take several hours
+    * Subsequent updates will be incremental only
+* Tagging example (work out which version first!):
+    * `git tag -a ver20.3.3 -m ver20.3.3 svn/tags/ver20.3.3`
+    * `git push qmk ver20.3.3`
+    * `git tag -a breaking_YYYY_qN -m breaking_YYYY_qN svn/tags/ver20.3.3`
+    * `git push qmk breaking_YYYY_qN`
+
+## Getting ChibiOS-Contrib
+
+* `git` Initialisation:
+    * `git clone [email protected]:qmk/ChibiOS-Contrib`
+    * `git remote add upstream https://github.com/ChibiOS/ChibiOS-Contrib`
+    * `git checkout -b chibios-20.3.x upstream/chibios-20.3.x`
+* Updating:
+    * `git fetch --all --tags --prune`
+    * `git checkout chibios-20.3.x`
+    * `git pull --ff-only`
+    * `git push origin chibios-20.3.x`
+    * `git tag -a breaking_YYYY_qN -m breaking_YYYY_qN chibios-20.3.x`
+    * `git push origin breaking_YYYY_qN`
+
+## Updating submodules
+
+* Update the submodules
+    * `cd $QMK_FIRMWARE`
+    * `git checkout develop`
+    * `git pull --ff-only`
+    * `git checkout -b chibios-version-bump`
+    * `cd lib/chibios`
+    * `git fetch --all --tags --prune`
+    * `git checkout breaking_YYYY_qN`
+    * `cd ../chibios-contrib`
+    * `git fetch --all --tags --prune`
+    * `git checkout breaking_YYYY_qN`
+* Build everything
+    * `cd $QMK_FIRMWARE`
+    * `qmk multibuild -j4`
+    * Make sure there are no errors
+* Push to the repo
+    * `git commit -am 'Update ChibiOS to XXXXXXXXX'`
+    * `git push --set-upstream origin chibios-version-bump`
+* Make a PR to qmk_firmware with the new branch