Add Maven Central release workflow and modernize build

[theJawnnybot]

Summary

• modernized project build to current plugin management style
• upgraded Gradle wrapper to 8.4
• upgraded AGP/Kotlin and dependency versions
• replaced JitPack setup with Maven Central publishing via com.vanniktech.maven.publish
• added tag-based GitHub Actions release workflow to run publishAndReleaseToMavenCentral
• updated CI workflow (checkout@v4, setup-java@v4, wrapper validation)
• updated README badges + dependency coordinates for Maven Central
• removed .jitpack.yml

Maven Central coordinates

implementation("com.commit451:coil-imagegetter:<version>")

Required repository secrets

• SONATYPE_NEXUS_USERNAME
• SONATYPE_NEXUS_PASSWORD
• SIGNING_PRIVATE_KEY
• SIGNING_PASSWORD

Validation

• ./gradlew build (passes locally)
• ./gradlew :coilimagegetter:tasks --all includes:
  • publishToMavenCentral
  • publishAndReleaseToMavenCentral