vpnmain.cgi: Mark MODP-1536 as broken, phase out MODP-2048

vpnmain.cgi: Mark MODP-1536 as broken, phase out MODP-2048

https://weakdh.org/imperfect-forward-secrecy-ccs15.pdf (released in
2015) recommends "to use primes of 2048 bits or larger", to which BSI's
techical guideline BSI-TR-02102 (https://www.bsi.bund.de/SharedDocs/Downloads/DE/BSI/Publikationen/TechnischeRichtlinien/TR02102/BSI-TR-02102.pdf?__blob=publicationFile&v=5)
concurs. The latter also recommends not to use DH groups comprising of
less than 2000 bits after 2022, and shift to 3000 bit DH groups earlier
as a precaution.

According to RFC 3526, section 8, MODP-1536 provides an estimated
security between 90 and 120 bits, a value that can be reasonably
considered broken today, as it has been so for other types of
cryptographic algorithms already, and per section 2.4 in the
aforementioned paper, breaking 1024-bit DH is considered feasible for
the NSA in 2015, which does not inspire confidence for MODP-1536 in
2022.

Therefore, this patch suggests to mark MODP-1536 as broken, since it
de facto is, and tag MODP-2048 as weak. The latter is also removed from
the default selection, so newly created VPN connections won't use it
anymore, to follow BSI's recommendations of using DH groups >= 3000 bits
in 2022 and later.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Acked-by: Michael Tremer <michael.tremer@ipfire.org>