committers-guide: Switch license to SPDX only

[ziaee]

Summary

Matching what we do on the licensing guide [0].
Should we also switch the link from the internal/licensing-policy [1]
to the licensing policy in the docs tree [0]? What's going on here?

[0]: https://docs.freebsd.org/en/articles/license-guide/
[1]: https://www.freebsd.org/internal/software-license/

[lwhsu]

I personally think this is a good direction and also what currently in style(9). We have so many copy-n-paste license in the tree, with more or less issues.

[ziaee]

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