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/web/auth/oauth2_provider.go

@@ -482,6 +482,7 @@ func OIDCWellKnown(ctx *context.Context) {
 	ctx.Data["OidcIssuer"] = jwtRegisteredClaims.Issuer // use the consistent issuer from the JWT registered claims
 	ctx.Data["OidcBaseUrl"] = strings.TrimSuffix(setting.AppURL, "/")
 	ctx.Data["SigningKeyMethodAlg"] = oauth2_provider.DefaultSigningKey.SigningMethod().Alg()
+	// FIXME: no need to use a Golang template to render JSON, just build the JSON response directly in the future
 	ctx.JSONTemplate("user/auth/oidc_wellknown")
 }