Upgrade Gradle wrapper to 9.6.1; fix Gradle-10 space-assignment deprecation

[serhiy-bzhezytskyy]

Fixes #599.

Bumps the Gradle wrapper 9.1.0 → 9.6.1 (current stable) via ./gradlew wrapper --gradle-version 9.6.1, and fixes the one Gradle-10 deprecation it surfaces.

Changes (5 files):

• gradle/wrapper/{gradle-wrapper.jar,gradle-wrapper.properties}, gradlew, gradlew.bat — standard wrapper task output for 9.6.1.
• src/build.gradle — group "org.apache.lucene" → group = "org.apache.lucene" (space-assignment is deprecated, removed in Gradle 10).

Verification (local; CI here is Python-only so it won't run Gradle):

• ./gradlew --version → Gradle 9.6.1
• ./gradlew jar / ./gradlew tasks --all → BUILD SUCCESSFUL
• ./gradlew jar --warning-mode all → 0 deprecations
• wrapper jar sha256 matches Gradle's official published checksum (76805e32…93f3).

Left out of scope (mentioned in #599 as optional follow-ups): pinning distributionSha256Sum and adding wrapper-validation to CI.

[mikemccand]

Woops. How come nothing was angry about the missing =?

[serhiy-bzhezytskyy]

(threading my earlier answer here where it belongs) Groovy lets you call a method without parens, so group "org.apache.lucene" was actually group("...") — valid for years, so it never errored; it only became a deprecation (removed in Gradle 10), silent unless --warning-mode all.

[serhiy-bzhezytskyy]

Groovy lets you call a method without parens, so group "org.apache.lucene" was actually a method call — group("...") — not an assignment. Gradle supported that form for years, so it never errored; it only became a deprecation warning recently (removed in Gradle 10), and it's silent unless you build with --warning-mode all. That's why nothing complained — valid, just the legacy style.

[mikemccand]

Thank you @serhiy-bzhezytskyy -- I'd be happy to merge those follow-ons if you have them.