CompatHelper: add new compat entry for "Plots" at version "1"

[github-actions[bot]]

This pull request sets the compat entry for the Plots package to 1.

This is a brand new compat entry. Previously, you did not have a compat entry for the Plots package.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.

Note: Consider registering a new release of your package immediately after merging this PR, as downstream packages may depend on this for tests to pass.