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).

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-8.14.3-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.6.1-bin.zip
 networkTimeout=10000
 validateDistributionUrl=true
 zipStoreBase=GRADLE_USER_HOME