Note Proton Calendar's source availability

[jonaharagon]

Changes proposed in this PR:

• https://discuss.privacyguides.net/t/proton-calendar-is-not-open-source-mobile/14656/8?u=jonah

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[privacyguides-bot]

This pull request has been mentioned on Privacy Guides. There might be relevant details there:

https://discuss.privacyguides.net/t/proton-calendar-is-not-open-source-mobile/14656/9

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[privacyguides-bot]

This pull request has been mentioned on Privacy Guides. There might be relevant details there:

https://discuss.privacyguides.net/t/v3-20/16594/1