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>

util/linuxfw/nftables_runner_test.go

@@ -1066,7 +1066,7 @@ func checkSNATRule_nft(t *testing.T, runner *nftablesRunner, fam nftables.TableF
 	if chain == nil {
 		t.Fatal("POSTROUTING chain does not exist")
 	}
-	meta := []byte(fmt.Sprintf("dst:%s,src:%s", dst.String(), src.String()))
+	meta := fmt.Appendf(nil, "dst:%s,src:%s", dst.String(), src.String())
 	wantsRule := snatRule(chain.Table, chain, src, dst, meta)
 	checkRule(t, wantsRule, runner.conn)
 }