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>

wgengine/pendopen.go

@@ -102,7 +102,7 @@ var (
 	canonicalIPs = sync.OnceValue(func() (checkIPFunc func(netip.Addr) bool) {
 		// https://bgp.he.net/AS41231#_prefixes
 		t := &bart.Table[bool]{}
-		for _, s := range strings.Fields(`
+		for s := range strings.FieldsSeq(`
 			91.189.89.0/24
 			91.189.91.0/24
 			91.189.92.0/24