Fix incorrect handling of UNTIL if falling into DST change and some related improvements

[minichma]

Reproduce and fix #736. I.e. if UNTIL falls into a DST change with ambiguity, then recurrences could have missed, which is fixed with this PR.

Other improvements include:

• Remove implicit conversion operator from System.DateTime to CalDateTime, which could cause confusion and was a source of bugs.
• Remove AsUtc from CalDateTime.GetHashCode() to avoid timezone calculations
• Avoid UTC conversion when comparing two CalDateTime if they have the same TzId

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
Ical.Net/DataTypes/CalDateTime.cs	25%	0 Missing and 3 partials ⚠️
...alization/DataTypes/RecurrencePatternSerializer.cs	0%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (75%) is below the target coverage (80%). You can increase the patch coverage or adjust the target coverage.
❌ Your project check has failed because the head coverage (66%) is below the target coverage (80%). You can increase the head coverage or adjust the target coverage.

@@         Coverage Diff         @@
##           main   #738   +/-   ##
===================================
- Coverage    66%    66%   -0%     
===================================
  Files       105    105           
  Lines      4649   4633   -16     
  Branches   1151   1144    -7     
===================================
- Hits       3073   3054   -19     
+ Misses     1142   1139    -3     
- Partials    434    440    +6     

Files with missing lines	Coverage Δ
Ical.Net/Evaluation/RecurrencePatternEvaluator.cs	91% <100%> (-<1%)	⬇️
...alization/DataTypes/RecurrencePatternSerializer.cs	72% <0%> (+1%)	⬆️
Ical.Net/DataTypes/CalDateTime.cs	86% <25%> (-4%)	⬇️

... and 1 file with indirect coverage changes