Revert "CI: switch to the "latest" LibreSSL"

This reverts commit aac6ff14dc8f5a5f951cd8285fdc56cf4f1c6f33.

The 2.5 is not compatible with LibreSSL 3.6.x.

(cherry picked from commit ef94861be2d2497d1df1151bfb038afee4fadeee)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/.github/matrix.py b/.github/matrix.py
index 14d3b5e..3b4ac0a 100644
--- a/.github/matrix.py
+++ b/.github/matrix.py
@@ -127,7 +127,7 @@
         "stock",
         "OPENSSL_VERSION=1.0.2u",
         "OPENSSL_VERSION=latest",
-        "LIBRESSL_VERSION=latest",
+        "LIBRESSL_VERSION=3.5.3",
 #        "BORINGSSL=yes",
     ]:
         flags = ["USE_OPENSSL=1"]