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).

.github/workflows/build.yml

@@ -15,16 +15,15 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          # 1.19.2 — Java 17
+          # 1.19.2 / 1.20.1 — Java 21 (plugin requires Java 21+; lolib jar is 21 bytecode)
-          - { mc: "1.19.2",  coordinate: "io.papermc.paper:paper-api:1.19.2-R0.1-SNAPSHOT", java: 17, release: 17, api: "1.19" }
+          - { mc: "1.19.2",  coordinate: "io.papermc.paper:paper-api:1.19.2-R0.1-SNAPSHOT", java: 21, release: 21, api: "1.19" }
-          # 1.20.1 — Java 17
+          - { mc: "1.20.1",  coordinate: "io.papermc.paper:paper-api:1.20.1-R0.1-SNAPSHOT", java: 21, release: 21, api: "1.20" }
-          - { mc: "1.20.1",  coordinate: "io.papermc.paper:paper-api:1.20.1-R0.1-SNAPSHOT", java: 17, release: 17, api: "1.20" }
           # 1.21 — Java 21
           - { mc: "1.21",    coordinate: "io.papermc.paper:paper-api:1.21-R0.1-SNAPSHOT",   java: 21, release: 21, api: "1.20" }
           # 1.21.11 — Java 21
           - { mc: "1.21.11", coordinate: "io.papermc.paper:paper-api:1.21.11-R0.1-SNAPSHOT", java: 21, release: 21, api: "1.21" }
-          # 26.1.2 / 26.2 — Java 25 (new-style paper-api build artifacts; may need a
+          # 26.1.2 / 26.2 — Java 25 (new-style paper-api build artifacts; requires the
-          # Gradle wrapper >= 8.14 if the daemon rejects Java 25 — bump if so).
+          # Gradle 9 wrapper + com.gradleup.shadow 9.x, which support running on Java 25).
           - { mc: "26.1.2",  coordinate: "io.papermc.paper:paper-api:26.1.2.build.74-stable", java: 25, release: 25, api: "1.21" }
           - { mc: "26.2",    coordinate: "io.papermc.paper:paper-api:26.2.build.97-stable",   java: 25, release: 25, api: "1.21" }
     steps:

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
         )
     }

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