docs: add note to v4 migration guide about node 18 min version

[adamstankiewicz]

Issue #, if available:

• https://github.com/amzn/style-dictionary/issues/1191
• https://github.com/amzn/style-dictionary/issues/1304

Description of changes:

Adds documentation regarding minimum supported version of Node.js 18+ to the Version 4 migration guide. Not sure how detailed the note should be; opted for brevity, highlighted in a note.

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