chore: migrate publishing to Maven Central and harden GitHub Actions

[kridai]

Summary

Mirrors sendgrid/java-http-client#151. OSSRH (oss.sonatype.org) has been shut down, so the current release path can't publish.

Release process (OSSRH -> Maven Central Portal)

• nexus-staging-maven-plugin 1.6.8 replaced with central-publishing-maven-plugin 0.8.0 (publishingServerId: central, autoPublish: true).
• Deploy job server-id: ossrh changed to central.
• Removed the deprecated org.sonatype.oss:oss-parent parent POM.
• Added a <developers> block (required by Central Portal), reusing Twilio API <api@twilio.com>. Please confirm that's the right contact.

SONATYPE_USERNAME / SONATYPE_PASSWORD must be Central Portal user tokens, not the old OSSRH credentials.

GitHub Actions hardening

• All actions pinned to commit SHAs, with Dependabot (github-actions and maven) keeping them current.
• Per-job permissions, ubuntu-x64 runners, and a repository_owner == 'sendgrid' guard.
• PR title lint uses twilio/sdk-actions/semantic-pr-title; GitHub Release uses twilio/sdk-actions/github-release (fetch-depth: 0, changelog-file: CHANGELOG.md, footer uses ${{ github.ref_name }}).
• Slack notifications and the Datadog metric are commented out (not on the allowlist).
• Removed update-dependencies.yml (PAT + direct push to main) in favour of Dependabot.

Testing

• YAML parses; mvn -Prelease -Dgpg.skip -DskipTests package produces the jar, -sources.jar and -javadoc.jar.
• An actual Central upload can only be verified on the next tag.

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