Bump appium from 2.11.5 to 2.15.0

Bumps [appium](https://github.com/appium/appium/tree/HEAD/packages/appium) from 2.11.5 to 2.15.0.
- [Release notes](https://github.com/appium/appium/releases)
- [Changelog](https://github.com/appium/appium/blob/master/packages/appium/CHANGELOG.md)
- [Commits](https://github.com/appium/appium/commits/appium@2.15.0/packages/appium)

---
updated-dependencies:
- dependency-name: appium
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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