fix: target Velocity 3.5.1 on Java 21

[zsigisti]

Summary

Targets Velocity 3.5.1 as the runtime platform, built on Java 21. This reverts the runtime requirement introduced in 794a875 ("require Velocity 4.1 build 8") so the plugin loads on Velocity 3.5.1.

Changes

gradle.properties only:

• velocity_api_version: 4.1.0 → 3.5.1
• velocity_snapshot: true → false (3.5.1 is a release, not a snapshot; fixes the dev-run server download)
• velocity_minimum_build: 8 → 615 (3.5.1's build per PaperMC's fill API)
• velocity_compile_api_version kept at 3.5.0 — velocity-proxy:3.5.1 is not published as a release (only SNAPSHOTs exist), and the 3.5.0-SNAPSHOT proxy is API-compatible with what runs on 3.5.1, so no source changes are needed.
• javaVersion unchanged (already 21).

Test plan

• ./gradlew clean shadowJar succeeds on openjdk 21.0.11 (prints Using Velocity API version: 3.5.1)
• Baked-in metadata.yml in the jar reads velocity_api_version: '3.5.1', velocity_minimum_build: 615
• Load the jar on a live Velocity 3.5.1 proxy and confirm the compatibility check passes