feat(airtable): use access token

[eddybrando]

Note: The PR Qiskit/gh-actions#54 needs to be merged first.

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Changes

With this PR we replace the Airtable API key with an Airtable access token.

Closes #3028

Implementation details

• AIRTABLE_API_KEY replaced by AIRTABLE_ACCESS_TOKEN
• AIRTABLE_ACCESS_TOKEN secret added to the repository (we can delete the AIRTABLE_API_KEY secret after this PR is merged)
• The Airtable access token is stored in the team's password manager as well.
• Cleaner local configuration via .env file (see changes in README)
• Grouped the keys in the .env.example file for easier reading

How to read this PR

• Look at the changes in the .env.example file
• Check the simple replacements of AIRTABLE_API_KEY with AIRTABLE_ACCESS_TOKEN
• Take a look at the changes pointed out in the README
• Check the script change in package.json