Run Flogger tests under JDK 22 instead of JDK 15.

I assume that our goal here was to use the latest STS.

We could also consider adding JDK 21 (as the latest LTS), and we could consider removing 11 (to keep 8 and 21 as LTS "endpoints"). I doubt it's worth thinking too hard about, but at least the bump from 15 to 22 seems uncontroversial. Hopefully it passes tests :)

As usual, we'll need to mess with the names of the CI jobs in [the branch-protection rules](https://github.com/google/flogger/settings/branches) after submission. (Copybara is happy to submit based only on its own requirements, ignoring GitHub's, but we want to make the requirements clear to external contributors.) We could in theory avoid that by picking consistent names instead of version numbers (e.g., "oldest LTS," "newest LTS," "newest STS"), but _shrug_.

RELNOTES=n/a
PiperOrigin-RevId: 663412398

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     name: "JDK ${{ matrix.java }}"
     strategy:
       matrix:
-        java: [8, 11, 15]
+        java: [8, 11, 22]
     runs-on: ubuntu-latest
     steps:
       # Cancel any previous runs for the same branch that are still running.