Remove all Google Calendar references from documentation

[Copilot]

This PR removes all mentions of Google Calendar from the codebase as requested. The Google Calendar links have been removed from:

1. README.md - Removed two instances:

   • The "Stay Updated" bullet point with Google Calendar link from the 2025 Cohort section
   • The Google Calendar link from the About DataTalks.Club footer section

2. after-sign-up.md - Removed one instance:

   • The bullet point about subscribing to the public Google Calendar for course announcements

All markdown formatting remains intact with no broken list items or structural issues. The changes are minimal and focused only on removing the Google Calendar references while preserving all other content and links.

After these changes, users can still stay updated through:

• Slack community (#course-mlops-zoomcamp channel)
• Telegram announcements channel
• YouTube playlist
• Newsletter and upcoming events on lu.ma

No cohort-specific folders (2022, 2023, 2024, 2025) contained Google Calendar references, so no changes were needed there.

Original prompt

Remove all mentions of Google Calendar from the codebase. Specifically:

• In README.md, remove the Google Calendar link from the cohort section and any community or update instructions.
• In after-sign-up.md, remove references to subscribing to the public Google Calendar for course announcements or updates.
• Review cohort folders such as cohorts/2023/README.md to ensure no Google Calendar references remain. (No explicit mention found in 2023, but check for others if present.)

These locations were confirmed by lexical search and may be incomplete, but these are the main visible places. After removal, ensure there are no broken list items or markdown formatting issues in the affected files.

This pull request was created as a result of the following prompt from Copilot chat.

Remove all mentions of Google Calendar from the codebase. Specifically:

• In README.md, remove the Google Calendar link from the cohort section and any community or update instructions.
• In after-sign-up.md, remove references to subscribing to the public Google Calendar for course announcements or updates.
• Review cohort folders such as cohorts/2023/README.md to ensure no Google Calendar references remain. (No explicit mention found in 2023, but check for others if present.)

These locations were confirmed by lexical search and may be incomplete, but these are the main visible places. After removal, ensure there are no broken list items or markdown formatting issues in the affected files.

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