[WEBSITE] Don't special case 0.22.3 and always use xvfb-run

Summary:
It will always work, worst case 2 instances will run which is a non
issue. Due to possible race fixed in D7913, use the -j1 flag to make
sure the man pages are built serially on older versions.

Test Plan:

make doc

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7916