Add ability to disable the new Altis version banner

When a new version of Altis is released, a banner is shown on the dashboard of sites that are not on that latest version (added by the `render_upgrade_widget()` function). For sites where the upgrade process is planned, scheduled, and managed by the delivery team this banner is not necessary because it's not actionable by the majority of the people who see it.

We'd like to be able to disable this banner on production environments, preferably via a filter. For example `if ( is_production() ) { add_filter( 'show_altis_upgrade_widget', '__return_false' ); }`.

Can a filter for this be added please? The alternative is to completely unhook the `maybe_render_header()` function which isn't exactly what we want to do.