CI: github: change "ubuntu-latest" to "ubuntu-20.04"

The ubuntu version was upgraded without us noticing, which meant
ssl=stock is 3.0 instead of 1.1.1.

Versions < 2.6 are not buildable without the deprecated openssl API so
let's rollback to a supported state.

Could be backported in every stable branch prior to 2.6.

(cherry picked from commit badff7fd93898bac2839fba0fb0165e4ed06c308)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
1 file changed