fix: RDATE and EXDATE property instances

[SebastianKrupinski]

• Resolves: [Bug]: iMIP emails don't take all EXDATEs into account when generating the email text #48732

Summary

• Added handling of RDATE and EXDATE property instances
• Added additional tests for RDATE and EXDATE
• Fixed spelling

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Backports requested where applicable (ex: critical bugfixes)

[st3iny]

Looks good! Thanks for adding tests.

[st3iny]

/backport to stable30

[ChristophWurst]

/backport to stable29

[backportbot]

The backport to stable29 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable29
git pull origin stable29

# Create the new backport branch
git checkout -b backport/48833/stable29

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick de221197

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/48833/stable29

Error: No changes found in backport branch

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[SebastianKrupinski]

/backport to stable29

This does not exist in 29. 30+ only