chore: Remove project.template_id foreignkey

[markstory]

I'd like to remove the ProjectTemplate models as we've never used them. Before the column and model can be dropped this foreign key needs to be removed.

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/1040_drop_fk_projecttemplate.py

for 1040_drop_fk_projecttemplate in sentry

--
-- Alter field template on project
--
SET CONSTRAINTS "sentry_project_template_id_67f0bec7_fk_sentry_pr" IMMEDIATE; ALTER TABLE "sentry_project" DROP CONSTRAINT "sentry_project_template_id_67f0bec7_fk_sentry_pr";
--
-- Moved project.template field to pending deletion state
--
-- (no-op)