Set connection timeout for curl

[yubiuser]

What does this PR aim to accomplish?:

Sets the --connection-timeout for the gravity curl (downloading adlists) to 10 seconds.
From the man pages:

       --connect-timeout <seconds>
              Maximum  time  in  seconds  that  you allow curl's connection to
              take.  This only limits the connection phase, so  if  curl  con‐
              nects  within the given period it will continue - if not it will
              exit.  Since version 7.32.0, this option accepts decimal values.

              If this option is used several times, the last one will be used.

              See also -m, --max-time.

This should speed up speed up gravity runs with a lot of inaccessible adlists. This was discussed on discourse: https://discourse.pi-hole.net/t/gravity-update-timeout/57391

The default timeout is 5 minutes.
https://github.com/curl/curl/blob/e43c3b3e3e6c2d58084d8c98f8e9640e09c0a05e/lib/connect.h#L47

#define DEFAULT_CONNECT_TIMEOUT 300000 /* milliseconds == five minutes */

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