chore(deps): bump apollo-server-core from 2.13.1 to 2.15.0

Bumps [apollo-server-core](https://github.com/apollographql/apollo-server) from 2.13.1 to 2.15.0.
- [Release notes](https://github.com/apollographql/apollo-server/releases)
- [Changelog](https://github.com/apollographql/apollo-server/blob/master/CHANGELOG.md)
- [Commits](https://github.com/apollographql/apollo-server/compare/apollo-server-core@2.13.1...apollo-server-core@2.15.0)

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