Increase DH Groups for VR from DH 18 to DH 21 or higher

[allanbuck]

The required feature described as a wish

When creating a VPN customer gateway, it would be nice if the IKE DH Group went higher then 18.

Can the DH GROUPS 19, 20, and 21 (even higher) be added to the drop down list. (ecp256, ecp384, ecp521). The used version of strongswan in 4.22 already supports these newer version so this change should be minimal.

Might also be good to remove or hide behind a setting the lower groups (2 and 5).

[boring-cyborg]

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[kiranchavala]

@allanbuck some the enhancements is already addressed via

#12193

cc @abh1sar

[abh1sar]

Yes, using the setting vpn.customer.gateway.excluded.dh.group some dh groups can be excluded from being shown or used. #12193

But support for higher DH groups would be good to have.