Make Alessio Caiazza BE maintainer

Trainee maintainer issue: #5356 (closed)

• I have worked at GitLab since Aug 2017 (I'm 29-releases old)
• I have authored on 270 MRs for gitlab-org https://gitlab.com/groups/gitlab-org/-/merge_requests?scope=all&utf8=%E2%9C%93&state=merged&author_username=nolith
• I maintain other components like gitlab-pages and gitlab-runner and am trainee maintainer for gitlab-workhorse.

Most of my trainee reviews were merged as is or with minor comments.

I feel ready to be a backend maintainer but would love to hear feedback that could help me improve if others disagree.

@gitlab-org/maintainers/rails-backend What do you think?

Closes #5356 (closed)