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>
diff --git a/.github/matrix.py b/.github/matrix.py
index 5c97efd..abd6caf 100644
--- a/.github/matrix.py
+++ b/.github/matrix.py
@@ -68,7 +68,7 @@
 
 # Ubuntu
 
-os = "ubuntu-latest"
+os = "ubuntu-20.04"
 TARGET = "linux-glibc"
 for CC in ["gcc", "clang"]:
     matrix.append(
@@ -156,7 +156,7 @@
 
 # ASAN
 
-os = "ubuntu-latest"
+os = "ubuntu-20.04"
 TARGET = "linux-glibc"
 for CC in ["gcc","clang"]:
     matrix.append(