Update MarkdownPost::getLatestPosts helper to sort using the DateTime object timestamp

[caendesilva]

Since the front matter data can differ in format, we should sort using the normalized DateTime object. By using the Unix timestamp we will always be able to sort using the proper order in time.

[codecov]

Codecov Report

Merging #1235 (13f8f45) into master (66f7c08) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##              master     #1235     +/-   ##
=============================================
  Coverage     100.00%   100.00%             
- Complexity      1315      2630   +1315     
=============================================
  Files            152       304    +152     
  Lines           3453      6910   +3457     
=============================================
+ Hits            3453      6910   +3457     

Impacted Files	Coverage Δ
packages/framework/src/Pages/MarkdownPost.php	100.00% <100.00%> (ø)
...ork/src/Framework/Concerns/TracksExecutionTime.php	100.00% <0.00%> (ø)
...packages/framework/src/Support/Models/RouteKey.php	100.00% <0.00%> (ø)
...c/Foundation/Concerns/BaseFoundationCollection.php	100.00% <0.00%> (ø)
...undation/Concerns/HandlesFoundationCollections.php	100.00% <0.00%> (ø)
...ramework/src/Console/Commands/BuildSiteCommand.php	100.00% <0.00%> (ø)
...amework/Features/Metadata/Elements/LinkElement.php	100.00% <0.00%> (ø)
...src/Framework/Exceptions/FileNotFoundException.php	100.00% <0.00%> (ø)
...elop/packages/framework/src/Facades/Filesystem.php	100.00% <0.00%> (ø)
...ramework/Views/Components/BreadcrumbsComponent.php	100.00% <0.00%> (ø)
... and 143 more

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