upkeep: bump lifecycle to "stable"

[jdhoffa]

This repository is very much in PROD, and hasn't received many serious changes in a while. It no longer feels "experimental" to me (and the "maturing" stage has itself been deprecated by lifecycle), so I think it makes sense to call this stable now.

Relates to RMI-PACTA/r2dii.data#375

[cjyetman]

@jdhoffa @jacobvjk this might be a priority if we want to have all r2dii packages "in production" as stable before launching pacta.loanbook

[jdhoffa]

Yeah that makes sense.
@MonikaFu let me know if you see any issues with bumping the lifecycle of r2dii.plot to "stable". Such a change would be purely "communicative" and not actually affect the code at all.
For what it's worth, given how few changes this repository has incurred in recent years, I think it's fair to call it "stable".

[MonikaFu]

I agree @jdhoffa !

[jdhoffa]

Awesome. Then @cjyetman would you mind opening a PR to bump the badge?