all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/privs_test.go

@@ -262,12 +262,10 @@ func maybeValidUID(id int) bool {
 		return true
 	}
 
-	var u1 user.UnknownUserIdError
-	if errors.As(err, &u1) {
+	if _, ok := errors.AsType[user.UnknownUserIdError](err); ok {
 		return false
 	}
-	var u2 user.UnknownUserError
-	if errors.As(err, &u2) {
+	if _, ok := errors.AsType[user.UnknownUserError](err); ok {
 		return false
 	}
 
@@ -281,12 +279,10 @@ func maybeValidGID(id int) bool {
 		return true
 	}
 
-	var u1 user.UnknownGroupIdError
-	if errors.As(err, &u1) {
+	if _, ok := errors.AsType[user.UnknownGroupIdError](err); ok {
 		return false
 	}
-	var u2 user.UnknownGroupError
-	if errors.As(err, &u2) {
+	if _, ok := errors.AsType[user.UnknownGroupError](err); ok {
 		return false
 	}