Update Go modules config

[SuperQ]

• Update minimum Go version to 1.20 (needed by dependencies).
• Remove inline /vendor dir (avoids repo bloat, sync issues).
• Enable monthly Go module dependabot updates.

[SuperQ]

My tool that compares Go mod version and module version detected this issue:

WARNING: Go mod version (1.18) is older than depencies (1.20)
- github.com/sapcc/go-api-declarations
- github.com/sapcc/go-bits

[majewsky]

The vendor dir stays. The other stuff is addressed in an ongoing feature branch.