Breaking: Drop support for old minor versions of Node 12 and 14

[bmish]

This matches ESLint's Node version support. It sets us up for:

• Using pure ESM dependencies (which are quickly becoming commonplace)
• Converting this package to ESM (see Sindre's post)
• Using other new Node features like node: prefix imports

Especially because this plugin is a dev-dependency for consumers, we can be more aggressive/strict here.

Originally discussed here: #151 (comment)

Part of v4 release (#120).

[aladdin-add]

not to against the change, just to clarify:

1. Sindre's packages require "node>=12". e.g. sindresorhus/is-unicode-supported@30ba81f#diff-7ae45ad102eab3b6d7e7896acd08c427a9b25b346470d7bc6507b6481575d519R16
2. if no build process, converting to esm is a breaking change, even it requires "node >=12.22.0" (esm cannot be required).

[bmish]

@aladdin-add Good points. Still, I think this PR is a worthwhile change.