update default repolinter ruleset

[willnorris]

This change updates our default ruleset in the following ways:

• move to new v2 syntax
• switch from json to yaml, mainly for comments support
• remove code of conduct rule, since it's no longer needed in each repo
• remove a few other rules, limiting it to the most basic that we are certain all repos should pass (have a README and LICENSE)
• move into repolinter-rulesets directory, which provides a place to put other extended rulesets that we may want to have apply to certain types of repos, such as those with source code versus simply documentation.

An example of how this would be expected to be used can be seen in some of our Twitter repos:

• Each repo would have a GitHub Action workflow: https://github.com/twitter/opensource-website/blob/main/.github/workflows/repolinter.yaml
• Each repo would have a minimal repolinter config which extends this default: https://github.com/twitter/opensource-website/blob/main/.github/repolinter.yaml. That is their extension point for adding or overriding individual rules from the default ruleset.
• Example output of running the action: https://github.com/twitter/opensource-website/actions/runs/1262622209

Relates to todogroup/working-hours#8