Move [L] license tags to SPDX identifiers

[svmhdvn]

We should be using standardized license identifiers[0] in all packages to distinguish between the various versions of licenses and clarify cases where dual licensing applies. From the FAQ[1]:

Q: How does one represent a file or package that is dual licensed (i.e., a license choice)?
A: SPDX license information can be represented using conjunctive or disjunctive regular expressions. For example, a file that is dual licensed under either the GPL-2.0 or MIT would be represented using the following disjunctive expression: (GPL-2.0 or MIT).

I can handle this task if you'd like, I love tedious and boring churn tasks like this.

[0] https://spdx.org/licenses/
[1] https://wiki.spdx.org/view/SPDX_FAQ

[rxrbln]

The currently used licenses are from historically only having a docent, like GPL, BSD, MIT, ... and we already support listing multiple, like GPL BSD … and started to have versioned one like GPL3.

Do you want to propose a patch or maybe better set of sed regex mass substitute our use cases?