Use `auto.middleware.tanstackstart` as middleware trace origin

[nicohrubec]

Currently we set manual.middleware.tanstackstart for any middleware span. This should be auto.middleware.tanstackstart

[linear]

JS-1622

[github-actions]

A PR closing this issue has just been released 🚀

This issue was referenced by PR #19137, which was included in the 10.39.0 release.