Fix license specification, to support cargo deny and cargo about

[cbeck88]

Many people are now using cargo deny to keep track of what licenses are in their dependency tree:

Currently, mbedtls crate has Apache-2.0/GPL-2.0+ listed as its license in Cargo.toml.

However, this is not a valid SPDX expression anymore -- GPL-2.0+ was deprecated in favor of GPL-2.0-or-later, in the latest versions of the spec.

https://spdx.org/licenses/

This results in errors with cargo deny:

error[unlicensed]: mbedtls = 0.8.1 is unlicensed
  ┌─ mbedtls 0.8.1
  │
2 │ name = "mbedtls"
  │         ^^^^^^^ a valid license expression could not be retrieved for the crate
3 │ version = "0.8.1"
4 │ license = "Apache-2.0 OR GPL-2.0+"
  │                                 - a GNU license was followed by a `+`

---

Suggestion: Change this to Apache-2.0/GPL-2.0-or-later to be help automated tools and downstream users

[Taowyoo]

Fixed by #263