chore: upgrade golangci-lint

[marcopaggioro]

Summary 💡

This PR aims to upgrade the version of golangci-lint.
This is required to alow the upgrade to the next major of version partially done in #662 .

Description 📝

Docs:

• https://golangci-lint.run/docs/product/changelog/
• https://golangci-lint.run/docs/product/migration-guide/
• https://github.com/maratori/golangci-lint-config/blob/main/.golangci.yml

Core activities:

• Moved functions and structs to comply with best practice order
• Filled non-exaustive switches
• Renamed packages with "reserved" name with "x" as suffix (like template -> templatex) as already done in previous packages
• Syntax improvement like slices.Contains
• Fix comments
• Removed unused functions and consts
• Use testing context in tests

Notes:

• .golangci.yml syntax changes from the 1.x version to the 2.x version. Version 2.x offers a cli utility and a dedicated docs page to help migration.
• Foreach new version I read the changelog and aligned the new lints with the golintci "golden config", where possible.

Breaking Changes 💔

Not detected.

Tests performed 🧪

All test suites.

Future work 🔧

Finish the Go major upgrade in #662