Pin MockBukkit to Maven Central 4.110.0 instead of jitpack snapshot

The floating jitpack snapshot v1.21-SNAPSHOT resolves to an ephemeral
git-described build whose jar/POM get evicted, breaking CI even on
unchanged commits. Switch to the equivalent stable Maven Central
coordinates (org.mockbukkit.mockbukkit:mockbukkit-v1.21:4.110.0).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: tastybento

pom.xml

@@ -141,9 +141,9 @@
     <dependencies>
         <!-- MockBukkit -->
         <dependency>
-            <groupId>com.github.MockBukkit</groupId>
-            <artifactId>MockBukkit</artifactId>
-            <version>v1.21-SNAPSHOT</version>
+            <groupId>org.mockbukkit.mockbukkit</groupId>
+            <artifactId>mockbukkit-v1.21</artifactId>
+            <version>4.110.0</version>
             <scope>test</scope>
             <exclusions>
                 <!-- Exclude MockBukkit's JUnit 6 transitive deps; we manage JUnit ourselves -->