chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/pmtud/icmp/icmp.go

@@ -25,7 +25,7 @@ func PathMTUDiscover(ctx context.Context, ip netip.Addr,
 		switch {
 		case err == nil:
 			return mtu, nil
-		case errors.Is(err, errTimeout) || errors.Is(err, ErrCommunicationAdministrativelyProhibited): // blackhole
+		case errors.Is(err, errTimeout) || errors.Is(err, errCommunicationAdministrativelyProhibited): // blackhole
 		default:
 			return 0, fmt.Errorf("finding IPv4 next hop MTU to %s: %w", ip, err)
 		}