accounts/external: convert signature v value to 0/1 (#20997)

This fixes an issue with clef, which already transforms the signature
to use the legacy 27/28 encoding.

Fixes #20994

accounts/accounts.go

@@ -129,6 +129,8 @@ type Wallet interface {
 	// about which fields or actions are needed. The user may retry by providing
 	// the needed details via SignHashWithPassphrase, or by other means (e.g. unlock
 	// the account in a keystore).
+	//
+	// This method should return the signature in 'canonical' format, with v 0 or 1
 	SignText(account Account, text []byte) ([]byte, error)
 
 	// SignTextWithPassphrase is identical to Signtext, but also takes a password