Ignore createBranch result explicitly to satisfy CheckReturnValue

The createBranch REST API now carries a @CheckReturnValue contract, so
ignoring its Response fails the errorprone build. Assign it to an
unused variable to make the intent explicit; the backup ref creation
behaviour is unchanged.

Change-Id: I089c879ca4893712b69985b749267d6e14691a48

src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java b/src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java
index d05eeaa..91336b0 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java
@@ -165,7 +165,8 @@
               ObjectId.toString(revWalk.parseCommit(ObjectId.fromString(refUpdate.oldRev)).getId());
 
           try {
-            createBranch.apply(project, IdString.fromDecoded(backupRef), input);
+            @SuppressWarnings("unused")
+            var unused = createBranch.apply(project, IdString.fromDecoded(backupRef), input);
           } catch (BadRequestException
               | AuthException
               | ResourceConflictException