build: bump ajv

[Mashal-m]

Description

Bump ajv to 6.12.3

TODO: Link to a JIRA ticket if one has been created for this feature/issue.
Use the following format for a link: [FEDX-100](https://openedx.atlassian.net/browse/FEDX-100)

TODO: Include a detailed description of the changes in the body of the PR.

TODO: Build a preview version of the documentation using gulp preview and add the generated link here.

Manual Test Plan

TODO: If your changes require manual tests, include instructions for any required manual tests,
and any manual testing that has already been performed.

Testing Checklist

• Manually test responsive behavior.
• Manually test right-to-left behavior.
• Manually test a11y support.

Non-testing Checklist

• Consider any documentation your change might need, and which users will be affected by this change.

Post-review

• Squash commits into discrete sets of changes with descriptive commit messages.

Reviewers

TODO: In this section, tag specific reviewers you know will want to have a look at this PR. Please
leave this section blank if you do not know who to tag, and we will automatically suggest reviewers
based on the code it touches.

TODO: You can use the checklist below for organization:

• @mention a reviewer here
• @mention a reviewer here

TODO: Consider not just code reviewers but also including reviewers from Docs, UX, Accessibility and Product, if applicable.

If you've been tagged for review, please check your corresponding box once you've given the 👍.