Use an SPDX identifier as license name in the POM

[TheMrMilchmann]

Using an SPDX identifier as license name in the POM is recommended by the official Maven documentation and allows tooling to automatically pick up license information.

I would usually file a PR for this, but I noticed that this project does take this configuration from another repository. Thus, I filed a PR there and this issue for tracking the update Kotlin/kotlinx.team.infra#27.