Adding team_member linter for linkedin field

What does this MR do?

There currently is no linter for the linkedin field on the team_member .yml files. We currently have 48 team member entries with an incorrectly formatted linkedin field, meaning the links don't render properly in the team page.

This MR introduces a simple linter - checking that the field contains neither an @ (assumed format for a username handle) character or / (assumed present in a URL or fragment). I don't think we need anything fancier right now, this will catch most-if-not-all of these errors!

Failing pipeline showing the error output: https://gitlab.com/gitlab-com/www-gitlab-com/-/jobs/7986126575

The linter is here: !136479 (diffs) and the rest of the MR is the team_member .yml file updates to have the correct values.