Use the job token as the default GITHUB_TOKEN

[nhedger]

Let's explore the possibility of using the job token (${{ github.token }}) as the default value for the token input.

inputs:
  token:
    description: GitHub Token
    required: true
    default: ${{ github.token }}

This would result in one less thing to consider when setting up the action, and we could remove the caveats section from the README.

To do

• Add a new token input to action.yaml
• Set its default value to ${{ github.token }}
• Remove the caveats section from README.md