[JENKINS-75400] Building tags containing slashes is broken for bitbucket server

Fix encoding of the URL for single tag endpoint. Tag name must be encoded but slash char.

Author: nfalco79

src/main/java/com/cloudbees/jenkins/plugins/bitbucket/server/client/BitbucketServerAPIClient.java

@@ -562,7 +562,8 @@ public BitbucketServerBranch getTag(@NonNull String tagName) throws IOException,
             .set("owner", getUserCentricOwner())
             .set("repo", repositoryName)
             .set("tagName", tagName)
-            .expand();
+            .expand()
+            .replace("%2F", "/");
 
         String response = getRequest(url);
         BitbucketServerBranch tag = JsonParser.toJava(response, BitbucketServerBranch.class);