Remove code that supports running the build with JDK 8

Addition of version catalog with use of java.util.Optional
already required JDK 11 in practice anyway.
Tests can still be run with JDK 8 though.

build-logic/src/main/kotlin/local/java-library.gradle.kts

@@ -7,11 +7,8 @@ plugins {
     id("net.ltgt.nullaway")
 }
 
-java.sourceCompatibility = JavaVersion.VERSION_1_8
-if (JavaVersion.current().isJava9Compatible) {
-    tasks.withType<JavaCompile>().configureEach {
-        options.release.set(java.targetCompatibility.majorVersion.toInt())
-    }
+tasks.withType<JavaCompile>().configureEach {
+    options.release.set(8)
 }
 
 dependencies {