Update to node 24 (#95)

* Use Node 24

Node 20 is deprecated and will stop working in June
https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

* README.md: Use checkout@v6 in example code

Author: gruenich

README.md

@@ -6,7 +6,7 @@ Any warnings or errors will be annotated in the Pull Request.
 ## Usage
 
 ```yml
-- uses: actions/checkout@v5
+- uses: actions/checkout@v6
 - uses: codespell-project/actions-codespell@v2
 ```
 

codespell-problem-matcher/action.yml

@@ -2,7 +2,7 @@ name: 'codespell problem matcher'
 author: 'Peter Newman'
 description: 'Shows codespell errors as annotation (with file and code line) in GitHub Actions'
 runs:
-  using: 'node20'
+  using: 'node24'
   main: 'index.js'
 branding:
   icon: 'search'