ci: remove boolean type from var definition [skip ci] (#19535)

Using a type like `boolean` in the variable definition scopes it to a stage,
like `def` would, but we need `isHotfix` in multiple stages.

Signed-off-by: Rado <radoslav@dhis2.org>

jenkinsfiles/stable

@@ -105,7 +105,7 @@ pipeline {
                     oldImageTag = env.DOCKER_IMAGE_TAG.replace("-rc", "")
 
                     // If version contains more than 2 dots... It's a hotfix.
-                    boolean isHotfix = oldImageTag.length() - oldImageTag.replace(".", "").length() > 2
+                    isHotfix = oldImageTag.length() - oldImageTag.replace(".", "").length() > 2
 
                     if (!isHotfix) {
                         oldImageTag = "$oldImageTag.0"