Bump Oj to v3.13.20 to fix illegal instruction errors

v3.13.19 was not enough to fix x86 compatability on older processors
because some SSE instructions were used by the compiler in other parts
of the code base. https://github.com/ohler55/oj/pull/806 disables SSE
4.2 optimizations by default.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/368656

Changelog: fixed