update upgrade.php for v1.3.6 -> 1.3.7

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | #554
| License       | MIT
| Doc PR        | -

 - [ ] update documentation links in upgrade pages
 - [ ] add documentation to github wiki or in src/docs/en/ (as it used to be)