chore: update eslint, rollup, webpack and their dependents

[hoobdeebla]

Most of these updates have been blocked in Renovate for a few months at minimum; this groups them in one place. Nonbreaking major changes:

Package	Change	Breaking Changes
coffee-loader	^0.9.0 -> ^2.0.0	require Node 10, webpack 5
dataloader	^1.4.0 -> ^2.0.0	none that affect usage in gatsby-plugin-mdx
enhanced-resolve	^4.2.0 -> ^5.7.0	none that affect usage in gatsby
friendly-errors-webpack-plugin	1.7.0-chalk2 -> ^2.0.0-beta.2	none
loader-utils	^1.4.0 -> ^2.0.0	require Node 8
postcss-loader	^4.1.0 -> ^5.2.0	require webpack 5
sass-loader	^10.1.1 -> ^11.0.1	require webpack 5
webpack-virtual-modules	^0.3.2 -> ^0.4.2	none

[wardpeet]

Thank you but we should be using renovate bot again to do these changes. For example webpack-virtual-modules broke subtle things in watch mode so that's a reason why we're still on the old one.

[hoobdeebla]

I understand where you're coming from @wardpeet, but at the same time, I opened this PR because Renovate wasn't updating its own PRs and because there are open Renovate PRs that have been ignored for months. There are packages that break Renovate updates not accounted for in the Renovate config, like webpack-virtual-modules (fixed in #30676).

Interestingly, there are also some Renovate PRs that have been stuck for no discernible reason and have to be manually re-ran. Some are not updating at all; some are still being force-pushed to approximately once a week, but their update list comments are not changing. Here are just a few of the PRs that are stuck:

#27092: readme unchanged since 2020-12-10
#27109: readme unchanged since 2020-11-18 and marked merge-on-green
#27134: readme unchanged since 2020-12-15
#27536: readme unchanged since 2021-01-20
#27898: readme unchanged since 2021-01-31 and marked merge-on-green
#28220: readme unchanged since 2021-02-27
#28508: readme unchanged since 2021-03-22

#28720: artifact update problem on 2021-03-11
#28722: artifact update problem on 2021-03-11
#28775: artifact update problem on 2021-03-11
#29281: artifact update problem on 2021-03-11

#28716: dirty yarn.lock on 2021-03-22
#29383: dirty yarn.lock on 2021-03-22
#29515: dirty yarn.lock on 2021-03-22