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>

cmd/k8s-operator/ingress-for-pg_test.go

@@ -1102,7 +1102,7 @@ func verifyTailscaledConfig(t *testing.T, fc client.Client, pgName string, expec
 			Labels:    pgSecretLabels(pgName, kubetypes.LabelSecretTypeConfig),
 		},
 		Data: map[string][]byte{
-			tsoperator.TailscaledConfigFileName(pgMinCapabilityVersion): []byte(fmt.Sprintf(`{"Version":""%s}`, expected)),
+			tsoperator.TailscaledConfigFileName(pgMinCapabilityVersion): fmt.Appendf(nil, `{"Version":""%s}`, expected),
 		},
 	})
 }