Moved C# SDK's packages ignore from SDK's .gitignore to project root

[rekhoff]

Description of Changes

Moves the root/sdks/csharp/packages/ directory from the root/sdks/csharp/.gitignore to root/.gitignore.
This is to resolve Issue #4207 , which was introduced during the DLLs generation rework.

/sdks/csharp/packages/ should be ignored by git, to prevent locally generated DLLs from accidentally getting added to the repo.

Root cause:
When Unity imports a package by URL, it looks at the .gitignore list and filters by it, causing the entire packages directory to not be imported. In earlier versions of Unity (like 2022.3 where this was initially tested), this was not a problem because Unity would allow us to do a dotnet restore. In modern version of Unity, this is no longer allowed as all imported packages are considered immutable.

Because the /sdks/csharp/ is cloned into com.clockworklabs.spacetimedbsdk, if we move at what level the root/sdks/csharp/packages/ is ignored at, we can prevent the accidental inclusion of the DLLs from developers in the SpacetimeDB repo, while ensuring com.clockworklabs.spacetimedbsdk does not contain a reference to ignore the packages directory.

API and ABI breaking changes

No API or ABI changes

Expected complexity level and risk

1

If you answered more than a 2, explain what is complex about the PR,
and what other components it interacts with in potentially concerning ways. -->

Testing

• Tested removing the packages directory from the .gitignore list on a custom branch of com.clockworklabs.spacetimedbsdk to observe the behavior in both Unity 6000.3.2f1 and 2002.3.62f2.

[bfops]

This works for me. The alternative would be to un-ignore the directory during the release flow itself, but I think this works well too.