fix(dav): Convert dates without tz drift

[ChristophWurst]

• Ref: feat(dav): dispatch out-of-office started and ended events #41440 (review)

Summary

If you do not leave at UTC, there is an offset for you timezone. Selecting a date in a local timezone and getting the date in UTC means there is a potential drift for times within the timezone offset.

This adjusts my bodge from #41340 where I add two days to the end to work around the drift.

How to test

1. Open /settings/user/availability
2. Enter an absence from November 10th to November 20th
3. Open the console and network tab
4. Click Save

On master: absence is set for Nov 9th to Nov 19th
Here: absence is set for Nov 10th to Nov 20th

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[ChristophWurst]

/backport to stable28

[st3iny]

Tested and works.