Bump mittwald/api-client from 2.1.75 to 2.1.87

Bumps [mittwald/api-client](https://github.com/mittwald/api-client-php) from 2.1.75 to 2.1.87.
- [Release notes](https://github.com/mittwald/api-client-php/releases)
- [Commits](mittwald/api-client-php@v2.1.75...v2.1.87)

---
updated-dependencies:
- dependency-name: mittwald/api-client
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>