[3.6] bpo-34399: 2048 bits RSA keys and DH params (GH-8762) (GH-8764)

[3.6] bpo-34399: 2048 bits RSA keys and DH params (GH-8762) (GH-8764)

Downstream vendors have started to deprecate weak keys. Update all RSA keys
and DH params to use at least 2048 bits.

Finite field DH param file use RFC 7919 values, generated with

    certtool --get-dh-params --sec-param=high

Signed-off-by: Christian Heimes <christian@python.org>.
(cherry picked from commit 88bfd0bce05043f658e50addd21366f317995e35)

Co-authored-by: Christian Heimes <christian@python.org>