Don't run the tests anymore on Node 12

[arcanis]

What's the problem this PR addresses?

Yarn 4 won't support Node 12.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.

[merceyz]

Shouldn't we set the lowest version to the first LTS release (14.15.0)?

A few more places to update

• berry/constraints.pro

  Line 49 in 63368a6

  gen_enforced_field(WorkspaceCwd, 'engines.node', '>=12 <14 || 14.2 - 14.9 || >14.10.0').

• berry/packages/vscode-zipfs/rollup.config.js

  Line 27 in dceb8fc

  target: `node12`,

• berry/packages/yarnpkg-builder/sources/commands/build/bundle.ts

  Line 131 in e324801

  target: `node12`,

• berry/packages/yarnpkg-builder/sources/commands/build/plugin.ts

  Line 123 in e05cf25

  target: `node12`,

• berry/packages/yarnpkg-core/rollup.config.js

  Line 44 in dceb8fc

  target: `node12`,

• berry/packages/yarnpkg-pnp/rollup.config.js

  Line 45 in 54317ad

  target: `node12`,

• berry/packages/yarnpkg-pnp/rollup.config.js

  Line 72 in 54317ad

  target: `node12`,

• berry/packages/yarnpkg-pnp/rollup.config.js

  Line 100 in 54317ad

  target: `node12`,