Upgrade action runtime to Node.js 24

GitHub Actions is deprecating the Node 20 runtime, with Node 24
becoming the default in June 2026. This updates the action to run
on the node24 runtime.

- action.yml: change `using` from node20 to node24
- .nvmrc: update development version to v24
- package.json: update engines to >=24

Closes #680

.nvmrc

@@ -1 +1 @@
-v20
+v24

action.yml

@@ -125,7 +125,7 @@ outputs:
   slug_prefix:
     description: Random prefix added to generated report anchor slugs for this action run
 runs:
-  using: 'node20'
+  using: 'node24'
   main: 'dist/index.js'
 branding:
   color: blue

package.json

@@ -83,7 +83,7 @@
     "titleTemplate": "{title}"
   },
   "engines": {
-    "node": ">=20"
+    "node": ">=24"
   },
   "markdownlint-cli2": {
     "ignores": [