ax(mining): replace banned fmt import with ErrUnsupportedMiner in miner_factory

fmt is a banned import (AX §banned-imports). miner_factory.go used
fmt.Errorf for the unsupported-miner error path; the package already
provides ErrUnsupportedMiner() in errors.go, so use that instead and
drop the fmt import entirely.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/miner_factory.go

@@ -1,7 +1,6 @@
 package mining
 
 import (
-	"fmt"
 	"strings"
 	"sync"
 )
@@ -77,7 +76,7 @@ func (f *MinerFactory) Create(minerType string) (Miner, error) {
 
 	constructor, ok := f.constructors[name]
 	if !ok {
-		return nil, fmt.Errorf("unsupported miner type: %s", minerType)
+		return nil, ErrUnsupportedMiner(minerType)
 	}
 
 	return constructor(), nil