How to force oracle accept insufficient swap size?
You possibly can't, really, unless there is a documented option for that. As the error is marked as "Severity:IGNORABLE" it is not unlikely that there will be an option to ignore "ignorable" warnings and proceed. Check the installation documentation for this information.

If not, and the problem is that you have a fixed swap partition/volume that you cannot increase because there is no space available to grow it into, you could temporarily add more swap by adding a swap file with something like:

dd if=/dev/zero of=/tmp/additionalswap bs=1048576 count=2100
chmod 600 /tmp/additionalswap
mkswap /tmp/additionalswap
swapon /tmp/additionalswap


before the installation process. This assumes you have sufficient space in the filesystem /tmp is found in, choose another location if not.

Once done, turn the extra swap space off with swapoff /tmp/additionalswap and remove the file.