Remove jsr305; switch GuardedBy to com.google.errorprone (#1611)

- Remove single usage of @immutable without replacement
- Remove HttpClient's usages of @threadsafe without replacement
- Replace javax.annotation.concurrent.GuardedBy
  with com.google.errorprone.annotations.concurrent.GuardedBy

Also:
- Remove redundant final modifiers from members of a final class

---------

Co-authored-by: odenix <self@odenix.org>

build-logic/src/main/kotlin/pklKotlinBase.gradle.kts

@@ -44,7 +44,6 @@ kotlin {
     freeCompilerArgs.addAll(
       "-jvm-default=no-compatibility", // was: -Xjvm-default=all
       "-Xjdk-release=${buildInfo.jvmTarget}",
-      "-Xjsr305=strict",
     )
   }
 }