Bump pnpm to v8, node to v20

[askoufis]

Doing a bit of spring cleaning 🧹. Up first is pnpm and node. Nothing of note for node, but the new pnpm major came with a new lockfile format.

NOTE: Not dropping support for any specific version of node in this PR. We don't even officially have a supported version anyway (AFAIK), but I think it's safe to drop testing on 14 given it became EOL ~4 months ago. 16 just became EOL a few days ago, but happy to keep it around for now.

This might also require a tweak to the branch protection since it's expecting a node 14 CI step to pass but it's now named node 16.

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[askoufis]

Can we keep the pnpm volta config, or does this cause an error if you don't enable have the volta envar set?

[mrm007]

We can, however it's prone to be forgotten (as was the case with this PR). Upgrading pnpm means you need to change packageManager, as well as the volta.pnpm key.

packageManager is more of a de facto standard, which is why I added it.