ci: fix multi-version build matrix (Gradle 9 + Java 21 for 1.19.x)

- gradle wrapper 8.14.3 -> 9.6.1 (supports Java 25 -> unblocks mc 26.x matrix)
- shadow plugin io.github.goooler.shadow 8.1.8 -> com.gradleup.shadow 9.6.1 (Gradle 9 compatible)
- fix ProcessResources expand null description under Gradle 9 strict typing
- matrix: 1.19.2/1.20.1 java 17 -> 21 (plugin requires Java 21; lolib jar is 21 bytecode)

Verified locally: default 1.20.4 quality gate, 1.19.2, 1.20.1, 1.21, 1.21.11 all green.
26.x builds now run on Gradle 9 in CI (needs JDK 25).

build.gradle.kts

@@ -16,7 +16,7 @@ buildscript {
 
 plugins {
     id("java")
-    id("io.github.goooler.shadow") version("8.1.8")
+    id("com.gradleup.shadow") version("9.6.1")
     id("xyz.jpenilla.run-paper") version("3.0.2")
     checkstyle
     pmd
@@ -244,7 +244,7 @@ tasks.withType<ProcessResources> {
     filesMatching(listOf("plugin.yml", "config.yml")) {
         expand(
             "version" to project.version,
-            "description" to project.description,
+            "description" to (project.description ?: ""),
             "apiVersion" to apiVersion
         )
     }