chore: Use relative ProjectReference paths instead of $(SolutionDir)

[0xced]

What does this PR do?

This pull request replaces all usage of $(SolutionDir) with relative paths instead.

Why is it important?

When using Testcontainers project directly from source, using $(SolutionDir) interferes since the $(SolutionDir) will be defined for the currently opened solution instead of Testcontainers.sln. Using relative paths work both for Testcontainers.sln and MyApp.sln. Using Testcontainers direclty from source is required to test bleeding edge feature such as WithReuse(true) which was introduced a few days ago.

[!NOTE]
directly from source means using

<ProjectReference Include="..\..\testcontainers-dotnet\src\Testcontainers.PostgreSql\Testcontainers.PostgreSql.csproj" />

instead of

<PackageReference Include="Testcontainers.PostgreSql" Version="3.7.0" />

Related issues

None

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