Prepare for merging from300 branch back by reverting the Gradle upgrade on master

[microbit-matt-hillsdon]

Suggested plan for moving back to using master/main @microbit-grace / @martinwork:

1. We merge all the android-partial-flashing-lib changes to master - via Merge changes through to 3.0.7 to master android-partial-flashing-lib#11
2. We merge this PR that reverts the Gradle update on master of microbit-android
3. We merge from300 into master (I’ve checked, this will merge cleanly after the Gradle revert) - via Merge changes through release 3.0.7 to master #50
4. We use GitHub’s UI to rename master to main for consistency with other projects

Commands to update a local copy:

  git branch -m master main
  git fetch origin
  git branch -u origin/main main
  git remote set-head origin -a

I made the same change to the partial flashing library.

6. Move pfLibrary as a git submodule or CI won't have a way to know what version to use.
   - I've done this in 08b8733
7. We tidy up docs to cover the current build process
8. We add a GitHub actions build replacing the CircleCI one that's currently failing (I'll do this)

Rationale for microbit-android

@martinwork said via email that "from300 is rooted in v2.8.8" and that "The master branch has had many commits since then, with which we don't want to merge" but on investigation there doesn't seem to be much change on master since the branches diverged.

There is no tag or GH release for v2.8.8 that I can see so working from git merge-base master from300 which is 5455c06 (Sam’s commit adding CircleCI support)

There are only 6 commits on master after 5455c06df78929afc0adced0ccb172c782df67f6

The 6 commits are all doc or issue template changes except one that updates Gradle
- The doc changes are OK/neutral. Grace and I will revisit the docs after merge to document the current build process.
- The Gradle change updates are from v5.1.1 to 6.5 (also updating the wrapper script)
- The Gradle version on the from300 branch has independently been upgraded to 8.2
- If you attempt to merge from300 to master Gradle is the only conflict. If you revert the Gradle upgrade on master first then it’s a clean merge.

So if we merge this PR then we get a clean merge of from300 back to master with no concerning commits on master that can see.

[martinwork]

Thanks @microbit-matt-hillsdon. I was forgetting that all the old changes were on branches. Go ahead!