fix: missing occurrences when there is a dst transition

[davec82]

using datetime zoned to generate occurrences, in case of a dst transition, the occurrence on time change day is missing

[Matsa59]

I got exactly this issue, could we consider this PR please ?

RRULE : every day AND ignoring the daylight saving : I want event at 10:00 every day

I store UTC date on db but I shift it on display.

[
  %{
    starts_at: ~U[2023-03-11 10:00:00Z],
    ends_at: ~U[2023-03-11 11:00:00Z],
    starts_at_timezone: "America/Montreal",
    ends_at_timezone: "America/Montreal"
  },
  %{
    starts_at: ~U[2023-03-13 09:00:00Z],
    ends_at: ~U[2023-03-13 10:00:00Z],
    starts_at_timezone: "America/Montreal",
    ends_at_timezone: "America/Montreal"
  }
]

If I removed the timezone it works BUT I have invalid hour obvs

In Quebec summer daylight saving is 12th march 2023 (timezone America/Montreal)

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (230963f) 96.55% compared to head (7fee206) 96.41%.

Files	Patch %	Lines
lib/cocktail/validation/shift.ex	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
- Coverage   96.55%   96.41%   -0.14%     
==========================================
  Files          21       21              
  Lines         493      502       +9     
==========================================
+ Hits          476      484       +8     
- Misses         17       18       +1     

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