Make this repo easier to get going with AKA this repo isn't abandoned

[jeffwidman]

A heads up from the core team: This repo isn't abandoned, or at least making it easier to run Dependabot as a standalone script/CI action isn't abandoned, despite the lack of recent activity here, and some breaking changes upstream in dependabot-core.

We're planning to get to making working with this repo easier over the next few months, but it may be another month or two until we get to it.

[joe-kimmel-vmw]

fwiw might be good to start with the ruby version - i just tried to follow the basic instruction rbenv install and got:

WARNING: ruby-2.6.6 is past its end of life and is now unsupported.
It no longer receives bug fixes or critical security updates.

ruby-build: using readline from homebrew
ruby-build: using gmp from homebrew

BUILD FAILED (macOS 13.1 using ruby-build 20230309)

I'm trying now with 2.7.7 but even that is only weeks away from EOL, I think.

[jeffwidman]

We've updated this repo's README with a deprecation note which points to a new repo we setup to demonstrate how to self-host Dependabot. As a result, we are archiving this repo.