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/syspolicy/policytest/policytest.go

@@ -89,12 +89,7 @@ func (pc policyChanges) HasChanged(v pkey.Key) bool {
 	return ok
 }
 func (pc policyChanges) HasChangedAnyOf(keys ...pkey.Key) bool {
-	for _, k := range keys {
-		if pc.HasChanged(k) {
-			return true
-		}
-	}
-	return false
+	return slices.ContainsFunc(keys, pc.HasChanged)
 }
 
 const watchersKey = "_policytest_watchers"