Automatically bump r base packages

[manics]

Proposed change

Follow-up from #1310 (comment)

We currently manually bump the versions of shiny_proxy_version and rsession_proxy_version. Can we automatically bump these?

Alternative options

• Do nothing
• Don't pin at all (though we're pinning all other automatically installed packages so why would this be an exception?)

Who would use this feature?

R users. R2d maintainers.

How much effort will adding it take?

Probably easiest to move these 2 dependencies into a dedicated requirements.txt file and let dependabot handle updates. Altenatively the freeze.py script could be updated.

Who can do this work?