perl: switch from backticks to qx()

To make it easier to find external command invocation in the source.

Also drop backticks from a comment.

To find external command invocations in Perl code use:
```sh
git grep -E "((exec|qx|open2|open3|system)\(|open\(.+-\|)" $(<perlfiles>)
```

Refs:
https://perldoc.perl.org/functions/qx
https://perldoc.perl.org/perlop#Simpler-Quote-Like-Operators

Closes #21994

scripts/mk-ca-bundle.pl

@@ -309,7 +309,7 @@ if(!$opt_n) {
 
     # If we have an HTTPS URL then use curl
     if($url =~ /^https:\/\//i) {
-        my $curl = `curl -V`;
+        my $curl = qx(curl -V);
         if($curl) {
             if($curl =~ /^Protocols:.* https( |$)/m) {
                 report "Get certdata with curl!";