package/openvpn: security bump to v2.6.20

See the release notes:
https://github.com/OpenVPN/openvpn/blob/v2.6.20/Changes.rst

This fixes 2 security issues:
- CVE-2026-40215:
    race condition in TLS handshake that could lead to leaking of packet
    data from a previous handshake under specific circumstances
- CVE-2026-35058:
    server ASSERT() on receiving a suitably malformed packet with a valid
    tls-crypt-v2 key

Signed-off-by: Titouan Christophe <titouan.christophe@mind.be>
(cherry picked from commit 1a4a04fe29)
Signed-off-by: Thomas Perale <thomas.perale@mind.be>

package/openvpn/openvpn.hash

@@ -1,3 +1,3 @@
 # Locally calculated after checking signature
-sha256  05cb5fdf1ea33fcba719580b31a97feaa019c4a3050563e88bc3b34675e6fed4  openvpn-2.6.16.tar.gz
+sha256  952ecee5b911a5353c0a6d40af62a7076c6dea1481ef204ce6d3f10481531315  openvpn-2.6.20.tar.gz
 sha256  edaef632cbb643e4e7a221717a6c441a4c1a7c918e6e4d56debc3d8739b233f6  COPYRIGHT.GPL

package/openvpn/openvpn.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-OPENVPN_VERSION = 2.6.16
+OPENVPN_VERSION = 2.6.20
 OPENVPN_SITE = https://swupdate.openvpn.net/community/releases
 OPENVPN_DEPENDENCIES = host-pkgconf libcap-ng
 OPENVPN_LICENSE = GPL-2.0