Add semgrep ruleset to validation schema

[wesley-dean-flexion]

Fixes #3152

Proposed Changes

Add a REPOSITORY_SEMGREP_RULESETS section to the JSON schema document used to validate .mega-linter.yml files.

Readiness Checklist

Author/Contributor

• Add entry to the CHANGELOG listing the change and linking to the corresponding issue (if appropriate)
• If documentation is needed for this change, has that been included in this pull request

Reviewing Maintainer

• Label as breaking if this is a large fundamental change
• Label as either automation, bug, documentation, enhancement, infrastructure, or performance

[nvuillam]

Many thanks @wesley-dean-flexion :) :)
I merge once the CI jobs are ok ^^

[wesley-dean-flexion]

@nvuillam yeah, I saw the email notifications about the linter failures come through but haven't had the opportunity to track them down. make tests ran and provided some linter warnings but they seemed unrelated to what I was doing. 🤷 I'll look into things later today.