Update to non-EA JDK 17 builds

and add JDK 8 to the matrix.

PiperOrigin-RevId: 399469056

Author: cushon

.github/workflows/ci.yml

@@ -29,19 +29,19 @@ jobs:
       fail-fast: false
       matrix:
         os: [ ubuntu-latest ]
-        java: [ 16, 14, 11 ]
+        java: [ 17, 14, 11 ]
         experimental: [ false ]
         include:
           # Only test on macos and windows with a single recent JDK to avoid a
           # combinatorial explosion of test configurations.
           - os: macos-latest
-            java: 16
+            java: 17
             experimental: false
           - os: windows-latest
-            java: 16
+            java: 17
             experimental: false
           - os: ubuntu-latest
-            java: 17-ea
+            java: 18-ea
             experimental: true
     runs-on: ${{ matrix.os }}
     continue-on-error: ${{ matrix.experimental }}