chore: update n8n to 2.21.7 and bump version to 2.55.0 (czlonkowski#797)

- Updated n8n-nodes-base from 2.20.4 to 2.21.2
- Updated n8n-core from 2.20.3 to 2.21.4
- Updated n8n-workflow from 2.20.0 to 2.21.1
- Updated @n8n/n8n-nodes-langchain from 2.20.4 to 2.21.4
- Rebuilt node database with 822 core nodes (542 AI tool variants, 86% docs)
- Refreshed community nodes: 1,029 total (911 verified + 118 unverified)
- Incremental community doc generation: 1,022/1,029 with README, 1,021/1,029 with AI summary
- Updated README badge (n8n 2.21.7) and node counts (1,851 total)
- Updated CHANGELOG with dependency changes

Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

Co-authored-by: Claude <noreply@anthropic.com>

Author: czlonkowski