OpenVPN: Replace existing Diffie-Hellman parameter with ffdhe4096

OpenVPN: Replace existing Diffie-Hellman parameter with ffdhe4096

Initial patch: https://git.ipfire.org/?p=people/ummeegge/ipfire-2.x.git;a=commit;h=2ccc799f8bd6a12c3edab5f1a89fab4d2cd05ea8

Minor adjustments to make it apply to the current state of "next", and
removal of chown operation in OpenSSL's LFS file, which would have lead
to the Diffie-Hellman group file being writable by nobody, for which
there is no necessity.

Fixes: #12632
From: Erik Kapfer <erik.kapfer@ipfire.org>
Signed-off-by: Peter Müller <peter.mueller@ipfire.org>