Update outdated Go version and dependencies from 1.15 to 1.21

[Copilot]

Problem

The repository had several outdated components that needed updating:

1. Go version mismatch: go.mod specified Go 1.15 (released 2020) while Dockerfile used Go 1.18 (released 2022)
2. Unsupported Go version: Go 1.15 is no longer supported by the Go team and lacks security updates
3. Stale dependencies: Package versions were locked to Go 1.15 compatibility
4. Outdated copyright: LICENSE file showed 2021 instead of current year 2025
5. Incorrect Docker path: Dockerfile referenced non-existent ./config/ directory instead of ./configs/

Changes

• Updated Go version: Upgraded from Go 1.15 to Go 1.21 LTS in both go.mod and Dockerfile
• Refreshed dependencies: Ran go mod tidy and go mod vendor to update all packages to Go 1.21 compatible versions
• Updated LICENSE: Changed copyright year from 2021 to 2025
• Fixed Dockerfile: Corrected config directory path from ./config/ to ./configs/
• Updated .gitignore: Added binary name to prevent accidental commits

Verification

• ✅ Project builds successfully with Go 1.21
• ✅ All existing tests pass
• ✅ Application starts and runs correctly
• ✅ Docker build configuration is fixed
• ✅ Vendor directory is synchronized with updated dependencies

Go 1.21 was chosen as the target version because it's a stable LTS release that provides modern Go features while maintaining broad compatibility and long-term support.

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