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>

tstest/integration/vms/vms_test.go

@@ -364,7 +364,7 @@ func (h *Harness) testDistro(t *testing.T, d Distro, ipm ipMapping) {
 		// starts with testcontrol sometimes there can be up to a few seconds where
 		// tailscaled is in an unknown state on these virtual machines. This exponential
 		// delay loop should delay long enough for tailscaled to be ready.
-		for count := 0; count < 10; count++ {
+		for range 10 {
 			sess := getSession(t, cli)
 
 			outp, err = sess.CombinedOutput("tailscale status")