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>

ipn/ipnlocal/netmapcache/netmapcache_test.go

@@ -275,7 +275,7 @@ var skippedMapFields = []string{
 func checkFieldCoverage(t *testing.T, nm *netmap.NetworkMap) {
 	t.Helper()
 
-	mt := reflect.TypeOf(nm).Elem()
+	mt := reflect.TypeFor[netmap.NetworkMap]()
 	mv := reflect.ValueOf(nm).Elem()
 	for i := 0; i < mt.NumField(); i++ {
 		f := mt.Field(i)