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/errors.go

@@ -10,9 +10,7 @@ import (
 
 var (
 	ErrNotPermitted                            = errors.New("ICMP not permitted")
-	ErrDestinationUnreachable                  = errors.New("ICMP destination unreachable")
-	ErrCommunicationAdministrativelyProhibited = errors.New("communication administratively prohibited")
-	ErrBodyUnsupported                         = errors.New("ICMP body type is not supported")
+	errCommunicationAdministrativelyProhibited = errors.New("communication administratively prohibited")
 	ErrMTUNotFound                             = errors.New("MTU not found")
 	errTimeout                                 = errors.New("operation timed out")
 )