Simplify licensing

[gmosx]

This PR simplifies the licensing to use Apache Version 2.0 only.
The Apache license is a more explicit version of the MIT license.
Similar to the AWS SDK project: https://github.com/awslabs/aws-sdk-rust

https://snyk.io/learn/apache-license/#apache-license-vs-mit

AFAIU this is the salient difference:

3. Grant of Patent License. Subject to the terms and conditions of this License, each Contributor hereby grants to You a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable (except as stated in this section) patent license to make, have made, use, offer to sell, sell, import, and otherwise transfer the Work, where such license applies only to those patent claims licensable by such Contributor that are necessarily infringed by their Contribution(s) alone or by combination of their Contribution(s) with the Work to which such Contribution(s) was submitted. If You institute patent litigation against any entity (including a cross-claim or counterclaim in a lawsuit) alleging that the Work or a Contribution incorporated within the Work constitutes direct or contributory patent infringement, then any patent licenses granted to You under this License for that Work shall terminate as of the date such litigation is filed.

The above looks useful to me.

FWIW, I am open to licensing under MIT instead, if there is a good argument.
I only want to avoid the dual licensing which complicates things with no reason.

cc @allanbrondumkr