chore: update changelog for v2.0.0 release

Added entries for v2.0.0, including Go generics support, unified authentication, enhanced documentation, CI improvements, dependency updates, and breaking changes. Credits contributors and links to the full changelog.

Author: jferrl

CHANGELOG.md

@@ -5,6 +5,37 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [v2.0.0] - 2025-08-16
+
+### Added
+
+- **Go Generics Support**: Introduced generic constraint `Identifier` interface supporting both `int64` App IDs and `string` Client IDs in a single `NewApplicationTokenSource` function
+- **Type-Safe Authentication**: Automatic type inference eliminates the need for separate functions while maintaining type safety
+- **Enhanced Documentation**: Official GitHub API references and JWT technical details while maintaining godoc compliance
+
+### Changed
+
+- **BREAKING**: Unified `NewApplicationTokenSource` function now uses Go generics to support both int64 App IDs and string Client IDs
+- **BREAKING**: Go version requirement bumped to 1.21+ (required for generics support)
+- Updated Go version to 1.25 in CI workflows and documentation
+- Improved CI workflow configurations with updated GitHub Actions
+
+### Fixed
+
+- Eliminated code duplication between App ID and Client ID authentication flows
+- Fixed go version usage from go.mod in GitHub Actions build (#12)
+
+### Dependencies
+
+- Added Dependabot configuration to keep dependencies up to date (#13)
+- Bumped `styfle/cancel-workflow-action` from 0.10.0 to 0.12.1 (#15)
+- Bumped `actions/checkout` from 4 to 5 (#18)
+- Bumped `codecov/codecov-action` from 4 to 5 (#19)
+
+**Contributors**: @jferrl, @grinish21
+
+**Full Changelog**: <https://github.com/jferrl/go-githubauth/compare/v1.2.1...v2.0.0>
+
 ## [v1.2.1] - 2025-08-08
 
 ### Fixed