use Java 21 in CI build matrix

Fixes #335

COPYBARA_INTEGRATE_REVIEW=#335 from sullis:sean/ci-jdk-21 d4361bd
PiperOrigin-RevId: 627018740

.github/workflows/ci.yml

@@ -14,15 +14,15 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-latest ]
-        java: [ 15, 11, 8 ]
+        java: [ 21, 11, 8 ]
         # Only test on macos and windows with a single recent JDK to avoid a
         # combinatorial explosion of test configurations.
         # Most OS-specific issues are not specific to a particular JDK version.
         include:
           - os: macos-latest
-            java: 15
+            java: 21
           - os: windows-latest
-            java: 15
+            java: 21
     runs-on: ${{ matrix.os }}
     steps:
       # Cancel any previous runs for the same branch that are still running.