Bump btcpayserver/btcpayserver-greenfield-php in /modules/btcpay

Bumps [btcpayserver/btcpayserver-greenfield-php](https://github.com/btcpayserver/btcpayserver-greenfield-php) from 2.8.0 to 2.8.1.
- [Release notes](https://github.com/btcpayserver/btcpayserver-greenfield-php/releases)
- [Commits](btcpayserver/btcpayserver-greenfield-php@v2.8.0...v2.8.1)

---
updated-dependencies:
- dependency-name: btcpayserver/btcpayserver-greenfield-php
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>