chore(build): update Gradle wrapper to 9.8.1 and Kotlin to 2.4.21 (#6301)

Gradle 9.8.1 fixes three high-severity advisories that affect Gradle
9.0.0 to 9.8.0 (GHSA-mvvg-497x-hmj8, GHSA-xwqc-3h47-hg64,
GHSA-j5m7-59rp-24f5). Kotlin 2.4.21 is a bug-fix release for 2.4.20;
the app's Kotlin compiler output is byte-identical to the 2.4.20 build.

V2rayNG/gradle/libs.versions.toml

@@ -2,7 +2,7 @@
 agp = "9.4.1"
 desugarJdkLibs = "2.1.5"
 gradleLicensePlugin = "0.9.91"
-kotlin = "2.4.20"
+kotlin = "2.4.21"
 coreKtx = "1.19.1"
 junit = "6.1.3"
 junitVersion = "1.3.0"

V2rayNG/gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
 #Thu Nov 14 12:42:51 BDT 2024
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-9.8.0-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.8.1-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists