Update grunt.

[rosiel]

GitHub Issue: Dependabot warnings (3 of them)

What does this Pull Request do?

In package.json, update the grunt version.

This does not solve all the problems, because we have a included a whole darn npm package within this repo, here, that hasn't been updated in a good many years.

What's new?

• Changes version of grunt.
• Does this change require documentation to be updated? don't think so.
• Does this change add any new dependencies? don't think so.
• Does this change require any other modifications to be made to the repository
  (i.e. Regeneration activity, etc.)? don't think so.
• Could this change impact execution of existing code? don't think so.

How should this be tested?

• I do not know, I am not a theme developer.

Additional Notes:

Any additional information that you think would be helpful when reviewing this
PR.

Interested parties

Tag (@ mention) interested parties or, if unsure, @Islandora/committers