oauth: Skip call-count test for libcurl 8.20.0

oauth: Skip call-count test for libcurl 8.20.0

The call-count test in 001_server.pl runs into a recent upstream
regression in Curl:

    https://github.com/curl/curl/issues/21547

The symptom is high CPU usage on some platforms during OAuth HTTP
requests. But it looks like the fix is on track for a June 2026 release,
as part of Curl 8.21.0, so just skip the test if we happen to be using
the broken version.

Reported-by: Andrew Dunstan <andrew@dunslane.net>
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Tested-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAOYmi%2B%3DyrwMSsHuNJ1V14isA4iSix5Xb3P3VEp1X0BS61MdV4A%40mail.gmail.com
Backpatch-through: 18