Update how to become a maintainer timeframe

Why is this change being made?

In https://about.gitlab.com/handbook/engineering/workflow/code-review/#how-to-become-a-maintainer we mention:

The MR should be open for at least 24 hours to allow all available backend maintainers to raise their comments.

So I think 48 hours 5 working days should be a decent timeframe to raise any significant concerns and will prevent blocking people to become a maintainer when either some people are away or don't want to voice their opinion (this could be because existing maintainers may not be that familiar with the specific project).

Based on !45947 (comment 327096171)

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what

cc @gitlab-org/maintainers/rails-backend cc @gitlab-org/maintainers/frontend