fix(java): prepare GitHub Packages release

[0xRAG]

Description

Prepares the public Java release surface for the Fern SDK cutover tracked by CDPSDK-3952:

• keeps GitHub Packages as the only Java publishing target;
• removes the unused Maven Central workflow, staging repository, and JReleaser configuration;
• adds the Jackson JDK8 module and forward-compatible Spotless exclusions required by the Fern-generated source layout; and
• updates the Java release runbook and Make targets accordingly.

The legacy generated-source Spotless exclusion remains temporarily so this change is safe before the GHE-to-public cutover sync lands.

Tests

• cd java && ./gradlew build -x test generatePomFileForMavenJavaPublication
• cd java && ./gradlew tasks --all (verified only publishMavenJavaPublicationToGitHubPackagesRepository remains)
• make -C java help
• git diff --check

Checklist

• Updated the typescript README if relevant (not applicable: Java release configuration only)
• Updated the python README if relevant (not applicable: Java release configuration only)
• Added a changelog entry (not applicable: release configuration; the Java cutover release fragment is in the companion GHE documentation PR)
• Added e2e tests if introducing new functionality (not applicable: build and release configuration only)

Generated with Toshi

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

⚠️ Ignored Reviews (2)

Reviewer	Reason
sammccord	Old or outdated review
sammccord	Review missing MFA

[linear]

CDPSDK-3952

[cb-heimdall]

Review Error for sammccord @ 2026-09-02 20:06:34 UTC
User failed mfa authentication, see go/mfa-help