UTC data migration doesn't work in Linux (closes #20002)

[lauraneto]

Closes #20002

This pull request adjusts the UTC migration code so that if a local time zone id is in the IANA format, it is converted to its corresponding Windows Id.
This is required for SQL Server as the AT TIME ZONE implementation relies on a Windows mechanism to convert datetime values across time zones, and therefore only accepts Windows ids.

[Copilot]

Pull Request Overview

This pull request fixes a Linux compatibility issue in UTC data migration where IANA time zone IDs couldn't be used with SQL Server's AT TIME ZONE operator. The solution converts IANA IDs to Windows IDs for SQL Server while maintaining SQLite compatibility.

Key Changes

• Removed validation that only accepted system-available time zones
• Enhanced migration logic to handle IANA to Windows time zone ID conversion
• Restructured the migration to support both SQLite and SQL Server with appropriate time zone handling

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
SystemDateMigrationSettingsValidatorTests.cs	Deleted test file for removed validator
MigrateSystemDatesToUtc.cs	Enhanced migration logic with IANA/Windows timezone conversion and database-specific handling
UmbracoBuilder.Configuration.cs	Removed registration of deleted validator
SystemDateMigrationSettingsValidator.cs	Deleted validator that restricted timezone IDs to system-available ones

[AndyButland]

Thanks @lauraneto. This looks good to me from a code perspective (at least once you've considered the minor comments I left). Once you are happy with it in your testing, please can you also liaise for next week with @andr317c and @nikolajlauridsen to verify this does now work as expected now on SQL Linux and any other variations we need to support (e.g. SQL Azure)? Thanks.

[AndyButland]

Couldn't quite follow why it was necessary to remove this validation. Seems we still call TimeZoneInfo.FindSystemTimeZoneById in the migration, so if so, it might be good to validate this upfront when Umbraco boots?

[lauraneto]

I removed it because it doesn't really matter if the application recognizes the time zone or not (at least when using SQL Server), what matters is if the SQL Server itself recognizes it.

Edit:
Just to add, this is helpful for edge cases where your OS differs from the one running the database, or if you don't have have all timezones installed on your docker image for example.
For SQLite we validate it only at the time of use, during the migration.