Change rotatePrimaryCert type to primitive boolean (#3205)

Change the rotatePrimaryCert field in CreateOrUpdateRegistrarCommand
from object Boolean to primitive boolean. Since the field is initialized
to default false and used as a presence-based switch without tri-state
semantics, primitive boolean accurately reflects its behavior and avoids
unnecessary object wrapper overhead.

BUG= http://b/537308816

core/src/main/java/google/registry/tools/CreateOrUpdateRegistrarCommand.java

@@ -138,7 +138,7 @@ abstract class CreateOrUpdateRegistrarCommand extends MutatingCommand {
       description =
           "Used together with --cert_file when updating an registrar. "
               + "If set, current cert is saved as failover.")
-  private Boolean rotatePrimaryCert = Boolean.FALSE;
+  private boolean rotatePrimaryCert = false;
 
   @Nullable
   @Parameter(