Update github actions to resolve most node deprecation warnings

As is github tradition, workflows started yelling about running on a
node version that was getting sunset, so here we go again.

Relevant bumps:

- https://github.com/actions/checkout/releases/tag/v4.0.0
- https://github.com/actions/setup-dotnet/releases/tag/v4.0.0
- https://github.com/actions/cache/releases/tag/v4.0.0
- https://github.com/actions/setup-java/releases/tag/v4.0.0
- https://github.com/peter-evans/create-pull-request/releases/tag/v6.0.0
- https://github.com/dorny/test-reporter/releases/tag/v1.8.0

Notably, `actions/upload-artifact` is _not_ bumped to v4, although it
should be to resolve the node deprecation warnings, because it has more
breaking changes and bumping would break `dorny/test-reporter`
(see https://github.com/dorny/test-reporter/issues/363).

.github/workflows/report-nunit.yml

@@ -28,7 +28,7 @@ jobs:
     timeout-minutes: 5
     steps:
       - name: Annotate CI run with test results
-        uses: dorny/test-reporter@v1.6.0
+        uses: dorny/test-reporter@v1.8.0
         with:
           artifact: osu-test-results-${{matrix.os.prettyname}}-${{matrix.threadingMode}}
           name: Test Results (${{matrix.os.prettyname}}, ${{matrix.threadingMode}})