docs: update instruction for node and pnpm version (#144)

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## What type of PR is this? (check all applicable)

- [ ] 🍕 Feature
- [ ] 🐛 Bug Fix
- [x] 📝 Documentation Update
- [ ] 🎨 Style
- [ ] 🧑‍💻 Code Refactor
- [ ] 🔥 Performance Improvements
- [ ] ✅ Test
- [ ] 🤖 Build
- [ ] 🔁 CI
- [ ] 📦 Chore (Release)
- [ ] ⏩ Revert

## Description

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Please do not leave this blank 
This PR [adds/removes/fixes/replaces] the [feature/bug/etc]. 
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This PR update the node and pnpm version.

## Related Tickets & Documents
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#143

## Mobile & Desktop Screenshots/Recordings

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## Added tests?

- [ ] 👍 yes
- [ ] 🙅 no, because they aren't needed
- [ ] 🙋 no, because I need help

## Added to documentation?

- [ ] 📜 README.md
- [ ] 📓 [vc-kit doc site](https://uncefact.github.io/vckit/)
- [ ] 📕 storybook
- [ ] 🙅 no documentation needed

## [optional] Are there any post-deployment tasks we need to perform?


<!-- note: PRs with deleted sections will be marked invalid -->

Signed-off-by: Nam Hoang <[email protected]>

README.md

@@ -18,8 +18,8 @@ The vckit is built on top of the [Veramo](https://veramo.io/) agent framework. V
 
 ## Prerequisites
 
-- [Node.js](https://nodejs.org/en/) version 16.x
-- [pnpm](https://pnpm.io/) version 8.x
+- [Node.js](https://nodejs.org/en/) version 18.x
+- [pnpm](https://pnpm.io/) version 8.6.0
 
 ## Installation and Setup