CI: switch to the "latest" LibreSSL

LibreSSL-3.6.0 had some regression, it was fixed in 3.6.1, let us
switch back to the latest LibreSSL available
diff --git a/.github/matrix.py b/.github/matrix.py
index 76e0893..ea4c79f 100755
--- a/.github/matrix.py
+++ b/.github/matrix.py
@@ -129,7 +129,7 @@
         "stock",
         "OPENSSL_VERSION=1.0.2u",
         "OPENSSL_VERSION=latest",
-        "LIBRESSL_VERSION=3.5.3",
+        "LIBRESSL_VERSION=latest",
         "QUICTLS=yes",
 #        "BORINGSSL=yes",
     ]: