chore(release): bump project version to 2.1.0

[tcheeric]

Summary

Bumps the project to 2.1.0 and records the release in CHANGELOG.md.

A minor bump is the right increment under semantic versioning: #546 adds NIP-17 and NIP-59 support, which is purely additive, and deprecates the NIP-04 types without removing anything. No existing caller has to change.

The version also matches the @Deprecated(since = "2.1.0") annotations already carried by EncryptedDirectMessage and MessageCipher04.

Type of change

• chore - Build, CI, or tooling changes
• docs - Documentation only

What changed?

• Root pom.xml and the four module POMs: 2.0.8 → 2.1.0, applied with scripts/release.sh bump --version 2.1.0
• CHANGELOG.md: the entries written in feat: add NIP-17 private direct messages with NIP-59 gift wrapping #546 move from [Unreleased] into a dated [2.1.0] section

No source changes.

Stacked on #546

Merge #546 first. This branch is built on it, so its diff will show only these two commits once #546 lands. Merging this alone would ship a version bump describing features that are not present.

Breaking changes

None. NIP-04 is deprecated but fully functional; removal would be a major bump and a separate decision.

Testing

mvn -q clean verify

Exits 0 at the new version, with no failures and no new warnings across all four modules.

Not included

Publishing is deliberately left out of this PR, since it needs GPG and Sonatype credentials and should follow a merge, not precede one. The remaining steps are in the version uplift how-to:

scripts/release.sh tag --version 2.1.0 --push
scripts/release.sh publish

The BOM (nostr-java-bom 1.1.8) still pins 2.0.8 and needs a follow-up release to advertise 2.1.0; see dependency alignment. Until then the root POM's dependencyManagement overrides keep the reactor internally consistent.

Checklist

• PR title follows conventional commits
• Changes are focused and under 300 lines
• No new compiler warnings introduced
• CHANGELOG.md updated