Upgrade Kotlin to 2.1.21, KSP to 2.1.21-2.0.1 (bazelbuild#1323)

* Upgrade Kotlin to 2.1.21

* Update docs

Author: Bencodes

src/main/starlark/core/repositories/versions.bzl

@@ -57,18 +57,18 @@ versions = struct(
         sha256 = "a9f923be58fbd32670a17f0b729b1df804af882fa57402165741cb26e5440ca1",
     ),
     KOTLIN_CURRENT_COMPILER_RELEASE = version(
-        version = "2.1.20",
+        version = "2.1.21",
         url_templates = [
             "https://github.com/JetBrains/kotlin/releases/download/v{version}/kotlin-compiler-{version}.zip",
         ],
-        sha256 = "a118197b0de55ffab2bc8d5cd03a5e39033cfb53383d6931bc761dec0784891a",
+        sha256 = "1ba08a8b45da99339a0601134cc037b54cf85e9bc0edbe76dcbd27c2d684a977",
     ),
     KSP_CURRENT_COMPILER_PLUGIN_RELEASE = version(
-        version = "2.1.20-1.0.32",
+        version = "2.1.21-2.0.1",
         url_templates = [
             "https://github.com/google/ksp/releases/download/{version}/artifacts.zip",
         ],
-        sha256 = "9f74659d447d3df29cabe0c8ee068810af313a584c1bbdde821f9894127f26e0",
+        sha256 = "44e965bb067b2bb5cd9184dab2c3dea6e3eab747d341c07645bb4c88f09e49c8",
     ),
     ANDROID = version(
         version = "0.6.0",
@@ -125,25 +125,25 @@ versions = struct(
         sha256 = "02c62574631876a4e3b02a1820cb51167bb9cdcdea2381b2fa9d9b8b11c407c4",
     ),
     KOTLINX_SERIALIZATION_CORE_JVM = version(
-        version = "1.6.3",
+        version = "1.8.1",
         url_templates = [
             "https://repo1.maven.org/maven2/org/jetbrains/kotlinx/kotlinx-serialization-core-jvm/{version}/kotlinx-serialization-core-jvm-{version}.jar",
         ],
-        sha256 = "29c821a8d4e25cbfe4f2ce96cdd4526f61f8f4e69a135f9612a34a81d93b65f1",
+        sha256 = "3565b6d4d789bf70683c45566944287fc1d8dc75c23d98bd87d01059cc76f2b3",
     ),
     KOTLINX_SERIALIZATION_JSON = version(
-        version = "1.6.3",
+        version = "1.8.1",
         url_templates = [
             "https://repo1.maven.org/maven2/org/jetbrains/kotlinx/kotlinx-serialization-json/{version}/kotlinx-serialization-json-{version}.jar",
         ],
-        sha256 = "8c0016890a79ab5980dd520a5ab1a6738023c29aa3b6437c482e0e5fdc06dab1",
+        sha256 = "58adf3358a0f99dd8d66a550fbe19064d395e0d5f7f1e46515cd3470a56fbbb0",
     ),
     KOTLINX_SERIALIZATION_JSON_JVM = version(
-        version = "1.6.3",
+        version = "1.8.1",
         url_templates = [
             "https://repo1.maven.org/maven2/org/jetbrains/kotlinx/kotlinx-serialization-json-jvm/{version}/kotlinx-serialization-json-jvm-{version}.jar",
         ],
-        sha256 = "d3234179bcff1886d53d67c11eca47f7f3cf7b63c349d16965f6db51b7f3dd9a",
+        sha256 = "8769e5647557e3700919c32d508f5c5dad53c5d8234cd10846354fbcff14aa24",
     ),
     RULES_BAZEL_INTEGRATION_TEST = version(
         version = "0.26.0",