Go back to using Java 11 to generate snapshot Javadoc.

(i.e., roll back a small portion of cl/615151162)

It turns out that Javadoc snapshots [fail when using Java 21](#6790 (comment)).

RELNOTES=n/a
PiperOrigin-RevId: 615532809

.github/workflows/ci.yml

@@ -93,10 +93,10 @@ jobs:
     steps:
       - name: 'Check out repository'
         uses: actions/checkout@9bb56186c3b09b4f86b1c65136769dd318469633 # v4.1.2
-      - name: 'Set up JDK 21'
+      - name: 'Set up JDK 11'
         uses: actions/setup-java@5896cecc08fd8a1fbdfaf517e29b571164b031f7 # v4.2.0
         with:
-          java-version: 21
+          java-version: 11
           distribution: 'zulu'
           cache: 'maven'
       - name: 'Generate latest docs'