PMM-7 Maintenance update

[ademidoff]

This pull request makes significant updates to the project's CI configuration, code formatting, linting, and ownership files. The main goals are to modernize CI workflows, improve code quality enforcement, and update project ownership.

Key changes include:

CI/CD Workflow Modernization

• Updated .github/workflows/ci.yml to use newer GitHub Actions (e.g., actions/checkout, actions/setup-go, actions/cache) and switched runners to ubuntu-22.04. The Go version matrix and related logic were removed in favor of using the version from go.mod. Steps for formatting, linting, and caching are now more standardized and robust. Debug output is improved for easier troubleshooting. [1] [2] [3]
• Removed the .github/workflows/clean.yml workflow, which previously handled scheduled cache cleaning.

Linting and Formatting Configuration

• Reworked .golangci.yml to a new format with explicit linter enables/disables, improved rules, and formatter settings. This enhances consistency and clarity in code quality checks.
• Removed .golangci-required.yml (previously used for required linters) and added .golangci.bck.yml as a backup configuration for almost-all linters, with comments explaining exclusions. [1] [2]
• Updated the Makefile to use gofumpt and goimports for formatting all .go files, improving code style consistency.

Project Ownership

• Changed the default code owner in .github/CODEOWNERS from @artemgavrilov to @ademidoff.

Code Quality and Testing Improvements

• Improved test code in alertmanager/config_test.go by removing unnecessary address-of (&) usage, switching to require.NoError for better test failure reporting, and minor refactoring for clarity. [1] [2] [3] [4] [5] [6] [7] [8]
• Added a missing doc comment for AlertingConfig in alerting.go.
• Added //nolint:intrange comments to suppress linter warnings for loop index types in copy.go. [1] [2]
• Improved the slice comparison in copy_test.go to avoid using the unsafe package, making the test safer and more idiomatic.