refactor: serve the api specs as plain json (#38715)

The api specs were Go templates whose committed form was not a valid
swagger document, so `swagger-validate`, `generate-openapi.go` and
`.spectral.yaml` each worked around it. They are now plain json,
substituted at serve time.

Renaming them off `.tmpl` also stops `make fmt` rewriting them, which
used to bump their mtime and silently skip the next `make
generate-swagger`.

Also enables stricter spectral linting: extends `lint-swagger` to the
OpenAPI 3 spec, turns on `openapi-tags`, `operation-singular-tag` and
`operation-tag-defined`, adds a top-level `tags` array with descriptions
to the swagger input, and drops the redundant `repository` tag from
`POST /user/repos`.

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

routers/api/v1/repo/repo.go

@@ -277,7 +277,7 @@ func CreateUserRepo(ctx *context.APIContext, owner *user_model.User, opt api.Cre
 
 // Create one repository of mine
 func Create(ctx *context.APIContext) {
-	// swagger:operation POST /user/repos repository user createCurrentUserRepo
+	// swagger:operation POST /user/repos user createCurrentUserRepo
 	// ---
 	// summary: Create a repository
 	// consumes: