Compute declared versions in a static block

This method is called often enough (when computing minimum compatibility
versions) that the reflection and sort can be seen while profiling. This
commit addresses this issue by computing the declared versions exactly
once.

Relates #28661

Author: Scott Somerville

server/src/main/java/org/elasticsearch/Version.java

@@ -163,6 +163,10 @@ public class Version implements Comparable<Version> {
                 + org.apache.lucene.util.Version.LATEST + "] is still set to [" + CURRENT.luceneVersion + "]";
     }
 
+    private static class DeclaredVersionsHolder {
+        static final List<Version> DECLARED_VERSIONS = Collections.unmodifiableList(getDeclaredVersions(Version.class));
+    }
+
     public static Version readVersion(StreamInput in) throws IOException {
         return fromId(in.readVInt());
     }
@@ -412,10 +416,10 @@ public int compareTo(Version other) {
     public Version minimumCompatibilityVersion() {
         if (major >= 6) {
             // all major versions from 6 onwards are compatible with last minor series of the previous major
-            final List<Version> declaredVersions = getDeclaredVersions(getClass());
             Version bwcVersion = null;
-            for (int i = declaredVersions.size() - 1; i >= 0; i--) {
-                final Version candidateVersion = declaredVersions.get(i);
+
+            for (int i = DeclaredVersionsHolder.DECLARED_VERSIONS.size() - 1; i >= 0; i--) {
+                final Version candidateVersion = DeclaredVersionsHolder.DECLARED_VERSIONS.get(i);
                 if (candidateVersion.major == major - 1 && candidateVersion.isRelease() && after(candidateVersion)) {
                     if (bwcVersion != null && candidateVersion.minor < bwcVersion.minor) {
                         break;