Do a 'tech debt' release focusing on internal API improvements and other breaking changes

[fredemmott]

These are now tagged "bc break"; let's try and bundle them up together into one release soon. It's been nice to support the 4.33 series for so long.

[jjergus]

Should this include new linters? We have #291 and #284 pending, both of which are likely to cause a significant number of new errors in most projects. Also, any opinion on adding linters to default + all vs only all? (the 2 PRs differ on that, which probably makes sense since one is more contentious -- but it might also be reasonable to add everything to "all" only first, then "default" in a subsequent version?)

[fredemmott]

I think for now we should not add anything new to default, and continue adding everything to 'all', given the points raised in #287. I wouldn't consider adding new defaults to all to be part of this.

'defaults' should be revisited as part #287 or #13