Migrate build tooling to Java 25 bytecode target and Gradle 9

[rokatyy]

Summary

Companion to the nuclio Java runtime migration to Java 25 LTS (nuclio/nuclio#4367). This branch carries three commits — the pre-existing in-flight bump-java work, the original Java 17 bytecode migration, and a follow-up bumping that target to Java 25 once nuclio dropped armhf support. The SDK's public API (EventHandler, Context, Event, etc.) has zero deprecated/removed API usage and no runtime dependencies, so all three are build-tooling-only changes, not source changes:

Commit 1 — Update for newest version (pre-existing, bump-java):

• Bumps version 1.1.0 → 1.2.0.
• Bumps JUnit Jupiter 5.0.1 → 5.8.2.
• Replaces deprecated Gradle syntax: classifier → archiveClassifier, testCompile/testRuntime → testImplementation/testRuntimeOnly, baseName → archivesBaseName.
• Makes GPG signing conditional on signing properties being present.

Commit 2 — Migrate build tooling to Java 17 bytecode target and Gradle 9:

• Originally pinned to Java 17 as the lowest common denominator across nuclio's Java runtime images (armhf was capped at JDK 17 at the time).
• Adds a real Gradle 9.7.1 wrapper, drops the ancient junit-platform-gradle-plugin for Gradle's native test { useJUnitPlatform() }, moves archivesBaseName into the Gradle-9-compatible base {} block, replaces dead Travis CI with GitHub Actions.

Commit 3 — Bump SDK bytecode target to Java 25, drop the JDK 17 floor:

• nuclio's companion PR drops armhf Java support entirely rather than pinning to JDK 17 indefinitely, so this SDK no longer needs a lowest-common-denominator target.
• Targets Java 25 via a Gradle toolchain (java { toolchain { languageVersion = JavaLanguageVersion.of(25) } }) rather than a plain sourceCompatibility/targetCompatibility pin — this auto-provisions a JDK 25 compiler via the Foojay resolver (new settings.gradle), so contributors don't need JDK 25 pre-installed locally.
• CI matrix drops JDK 17 — compiling for --release 25 requires the build's own JVM to already be JDK 25+.
• Version stays at 1.2.0 — never actually published to Maven Central, so there's nothing to skip past.

Test plan

• Real ./gradlew build under the actual Gradle 9.7.1 wrapper, with the toolchain auto-downloading a JDK 25 — succeeds.
• Verified via that JDK 25 toolchain's own javap that the compiled class files are major version 69 (Java 25 bytecode), confirming the target took effect.
• Republish to Maven Central — needed before nuclio's Dockerfile SDK pin can be bumped off 1.1.0 (not blocking: old bytecode still loads fine on the new JVMs).

Known non-blocking follow-up

A few Gradle-10-forward deprecation warnings remain (archives configuration for artifact declaration, Groovy space-assignment syntax) — not blocking today since Gradle 10 doesn't exist yet, flagging so it doesn't surprise anyone later.

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