Support Java 21 builds

[wonjong-yoo]

This PR upgrades the gradle and pitest versions and modifies the code to support building with Java 21.

• upgrade gradle: 8.1.1 → 8.8 (Java 21 needs gradle >= 8.5. see: https://docs.gradle.org/8.5/release-notes.html#java-21)
• upgrade pitest: 1.14.2 → 1.16.3 (Java 21 needs pitest >= 1.15.0 see: https://stackoverflow.com/a/77261168)
• change RateLimitedInputStream:: MIN_RATE: 8192 → 16384 (see: https://bugs.openjdk.org/browse/JDK-8299336)