[ci skip] Populate test plugin api-version from mcVersion

closes #7216

test-plugin/build.gradle.kts

@@ -10,8 +10,14 @@ dependencies {
 }
 
 tasks.processResources {
-    inputs.property("version", project.version)
+    val apiVersion = rootProject.providers.gradleProperty("mcVersion").forUseAtConfigurationTime().get()
+        .split(".", "-").take(2).joinToString(".")
+    val props = mapOf(
+        "version" to project.version,
+        "apiversion" to apiVersion,
+    )
+    inputs.properties(props)
     filesMatching("plugin.yml") {
-        expand("version" to project.version)
+        expand(props)
     }
 }

test-plugin/src/main/resources/plugin.yml

@@ -3,5 +3,5 @@ version: ${version}
 main: io.papermc.paper.testplugin.TestPlugin
 description: Paper Test Plugin
 author: PaperMC
-api-version: 1.17
+api-version: ${apiversion}
 load: STARTUP