chore(Cross): [IOAPPX-282] Move Mixpanel URL into env file

[LazyAfternoons]

Short description

This PR moves the Mixpanel URL into our env file.

List of changes proposed in this pull request

• Adds the URL in .env.local and .env.production;
• Adds the constant in config.ts;
• Remove the hardcoded URL from mixpanel.ts.

How to test

Test if Mixpanel events are tracked properly.

[pagopa-github-bot]

Affected stories

• ⚙️ IOAPPX-282: Link API Mixpanel in file di env

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