Repair syntax in alertmanager['flags'] block

What does this MR do?

Corrects a syntax error in the alertmanager['flags'] block. Currently, activating it results in:

There was an error running gitlab-ctl reconfigure:

/etc/gitlab/gitlab.rb:1639: syntax error, unexpected tSTRING_BEG, expecting '}' 'storage.path' => '/var/opt/gi... ^ /etc/gitlab/gitlab.rb:1639: syntax error, unexpected =>, expecting end-of-input 'storage.path' => '/var/opt/gitlab/alertmanage... ^~

Premium customer (internal ticket & internal discussion) is seeing this problem on 12.7.6 and I saw it on 13.3.0.

Related issues

Checklist

See Definition of done.

• Changelog entry created. Not applicable for Documentation changes and minor changes.
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA, if applicable
• MR targeting master branch
• MR has a green pipeline on GitLab.com
• Equivalent MR/issue for CNG opened if applicable
• trigger-package has a green pipeline running against latest commit

Reviewer Checklist

In addition to above, reviewer must:

• Pipeline is green on dev.gitlab.org if the change is not touching documentation or internal cookbooks