Fix pitest dependency resolution by using stable runtime version (opensearch-project#5143)

The pitest RC runtime artifacts (pitest-command-line:1.19.0-rc.2) are
not published to Maven Central, causing dependency resolution failures.
Update pitestVersion to 1.22.1 (latest stable) and junit5PluginVersion
to 1.2.1 while keeping the Gradle plugin at 1.19.0-rc.2 for Gradle 9
compatibility.

Signed-off-by: Vamsi Manohar <reddyvam@amazon.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: vamsimanohar

core/build.gradle

@@ -38,11 +38,11 @@ repositories {
 
 pitest {
     targetClasses = ['org.opensearch.sql.*']
-    pitestVersion = '1.19.0-rc.2'
+    pitestVersion = '1.22.1'
     threads = 4
     outputFormats = ['HTML', 'XML']
     timestampedReports = false
-    junit5PluginVersion = '1.0.0'
+    junit5PluginVersion = '1.2.1'
 }
 
 dependencies {