Performance: Optimize memory footprint of document URL cache (closes #21055) (#21066)

* Optimize memory footprint of document URL cache.

* Update tests/Umbraco.Tests.UnitTests/Umbraco.Core/Services/DocumentUrlServiceTests.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Used properties, added some further comments.

* Fixed failing integration tests.

* Ensure no edge case exists where the culture code to language Id map isn't up to date with the newly created languages.

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 2 people