all: clean up minor issues found by staticcheck

Use bytes.Equal instead of comparing bytes.Compare results to zero,
avoid Yoda conditions, drop a redundant zero slice capacity, use
strings.ReplaceAll, build acme error strings with a strings.Builder,
and lowercase an error string in ocsp. No behavior changes.

Change-Id: I5d2b3fa5c241adc75e61be0f2d2d7af1eb9e8d66
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/803200
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

ssh/session_test.go

@@ -745,7 +745,7 @@ func TestSessionID(t *testing.T) {
 
 	s := <-serverID
 	c := <-clientID
-	if bytes.Compare(s, c) != 0 {
+	if !bytes.Equal(s, c) {
 		t.Errorf("server session ID (%x) != client session ID (%x)", s, c)
 	} else if len(s) == 0 {
 		t.Errorf("client and server SessionID were empty.")