Bump stechstudio/backoff from 1.3 to 1.4 in /modules/btcpay

Bumps [stechstudio/backoff](https://github.com/stechstudio/backoff) from 1.3 to 1.4.
- [Release notes](https://github.com/stechstudio/backoff/releases)
- [Commits](stechstudio/backoff@1.3...1.4)

---
updated-dependencies:
- dependency-name: stechstudio/backoff
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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