Use latest UnitsNet NuGet in samples

[angularsen]

Motivation

PR #1544 left the samples with a development-only package override for normal builds, while only the Official configuration used the published NuGet package. That override also pointed to the accidental pre103 version.

The samples should always exercise the package users install.

Changes

• Reference UnitsNet 6.0.0-pre021 centrally for all samples.
• Remove the configuration-specific package overrides.
• Update the custom quantity sample to implement ILinearQuantity, matching the current v6 aggregation APIs.

Validation

• Restored the sample solution with an empty-cache package resolution.
• Built Samples/Samples.slnx in Release successfully.
• Verified that no development VersionOverride or conditional UnitsNet package references remain.
• Ran git diff --check.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98%. Comparing base (42205dc) to head (6bf0e59).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #1712 +/- ##
========================================
- Coverage 98% 98% -1% 
========================================
Files 512 514 +2 Lines 22883 24037 +1154 ========================================
+ Hits 22611 23644 +1033 - Misses 272 393 +121 

Flag	Coverage Δ
net48	97% <ø> (?)

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