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/provider/purevpn/updater/servers.go

@@ -17,7 +17,7 @@ func (u *Updater) FetchServers(ctx context.Context, minServers int) (
 	servers []models.Server, err error,
 ) {
 	if !u.ipFetcher.CanFetchAnyIP() {
-		return nil, fmt.Errorf("%w: %s", common.ErrIPFetcherUnsupported, u.ipFetcher.String())
+		return nil, fmt.Errorf("IP fetcher %s does not support fetching any IP", u.ipFetcher.String())
 	}
 
 	const url = "https://d11a57lttb2ffq.cloudfront.net/heartbleed/router/Recommended-CA2.zip"