Remove .eslintignore workaround and update eslint to at least 7.31.0

[tjenkinson]

https://github.com/rollup/plugins/blob/6463b9635e20b8425599f14005b9efdd8f94ecd6/.eslintignore#L8-L9

can be removed now if we update eslint to at least 7.31.0 which includes my fix https://github.com/eslint/eslintrc/pull/42 in https://github.com/eslint/eslint/pull/14808

[shellscape]

(Don't skip that PR template please. We tell users that even maintainers have to use them ;))

[tjenkinson]

Didn’t think there was a template this fitted well(?) so went with a blank one and given it’s assigned to me though it was ok. I’ll follow the PR template when I open one