Remove UploadFile in favor of UploadProjectMarkdown to align to API structures

See #2088 (comment 2306618407) for additional information.

There are currently two functions (UploadFile and UploadProjectMarkdown) that do similar functions. While historically we would remove one of the functions as a breaking change , this would potentially cause some users to pin versions and not upgrade because they thought we removed support instead of just moving the function. To aid in discoverability, we will deprecate UploadFile and point users to the new function.

This issue is targeted at removing UploadFile completely. While this library doesn't follow GitLab release cycles, aligning this issue to the release of GitLab 18.0 is convenient for the maintainers because we already have some other breaking changes we make in that timing!

Migrating

Users migrating will need to do the following.

1. Update Projects.UploadFile to ProjectMarkdownUploads.UploadProjectMarkdown.