Deprecate project public_source field

[dglsparsons]

Summary

Deprecates the vercel_project.public_source and project data source public_source attributes now that Vercel no longer supports project-level public source/log visibility.

This also stops sending publicSource in create project payloads. For the resource, the deprecated Terraform value is preserved from config/state when reading so existing configurations do not hit perpetual diffs or inconsistent-result errors when the API omits the field.

Root cause

publicSource was removed from the Projects API after the source/log visibility backfill. Older provider behavior still sent the field and expected it back, so public_source = true could apply successfully and then read back as null.

Validation

• env GOCACHE=/private/tmp/tpv-go-build-cache-public-source go test ./vercel -run 'TestConvertResponseToProject|TestProject|TestDataSourceProject'
• env GOCACHE=/private/tmp/tpv-go-build-cache-public-source go test ./client -run '^$'

A broader go test ./client ./vercel was attempted before the rebase, but this sandbox cannot bind httptest ports and cannot access the local GPG agent for acceptance-style tests that create signed git commits.