Bump devpi-server from 6.14.0 to 6.15.0 in /server

Bumps [devpi-server](https://github.com/devpi/devpi) from 6.14.0 to 6.15.0.
- [Changelog](https://github.com/devpi/devpi/blob/main/doc/changelog.rst)
- [Commits](devpi/devpi@server-6.14.0...server-6.15.0)

---
updated-dependencies:
- dependency-name: devpi-server
  dependency-version: 6.15.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

server/requirements.txt

@@ -1,2 +1,2 @@
-devpi-server==6.14.0
+devpi-server==6.15.0
 devpi-web==4.3.0