[#1917] Fix CI failure setting up JDK 21

It's not available anymore from jdk.java.net

Author: DavideD

.github/workflows/build.yml

@@ -151,7 +151,7 @@ jobs:
           # even if we don't use these features, just enabling them can cause side effects
           # and it's useful to test that.
           - { name: "20", java_version_numeric: 20, jvm_args: '--enable-preview' }
-          - { name: "21", java_version_numeric: 21, from: 'jdk.java.net', jvm_args: '--enable-preview' }
+          - { name: "21", java_version_numeric: 21, jvm_args: '--enable-preview' }
           - { name: "22-ea", java_version_numeric: 22, from: 'jdk.java.net', jvm_args: '--enable-preview' }
           - { name: "23-ea", java_version_numeric: 23, from: 'jdk.java.net', jvm_args: '--enable-preview' }
     steps: