ci: configure gpg in the GNUPGHOME that setup-java v6 creates

[zimeg]

This pull request fixes the release workflow, which fails in the "Configure GPG" step after the actions/setup-java v6 bump (#1640).

• setup-java v6 imports the signing key into an isolated temporary keyring and exports GNUPGHOME (actions/setup-java#1214), so ~/.gnupg no longer exists and echo ... >> ~/.gnupg/gpg.conf exits with "No such file or directory".
• The loopback pinentry settings are now written to ${GNUPGHOME}/gpg.conf and ${GNUPGHOME}/gpg-agent.conf, the keyring setup-java imported the key into.
• The deprecated server-username, server-password, and gpg-passphrase inputs are renamed to their *-env-var replacements with the same values. With MAVEN_GPG_PASSPHRASE as the passphrase variable, setup-java adds nothing to settings.xml, and -Dgpg.passphrase continues to work with maven-gpg-plugin 1.6.

Testing

• After merging, run the "Release" workflow from main and confirm "Configure GPG" passes and the artifacts are signed and published.

Category (place an x in each of the [ ])

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you agree to those rules.

🤖 Generated with Claude Code

[zimeg]

📝 A comment of changes needed most urgent!

[zimeg]

👁️‍🗨️ note: This issue was raised in release most recent:

https://github.com/slackapi/java-slack-sdk/actions/runs/36782832971/job/110117712387

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.90%. Comparing base (b0eefe7) to head (bdc0d00).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1649   +/-   ##
=========================================
  Coverage     72.90%   72.90%           
  Complexity     4593     4593           
=========================================
  Files           483      483           
  Lines         14565    14565           
  Branches       1520     1520           
=========================================
  Hits          10618    10618           
  Misses         3044     3044           
  Partials        903      903           

Flag	Coverage Δ
jdk-14	72.90% <ø> (ø)

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