fix: Use license instead of license-file in Cargo.toml (microsoft#464)

Since Regorus is MIT-licensed, it would be better to specify `license = "MIT"` rather than using `licene-file` to point to the MIT license file.
Some tools do not support parsing of `license-file`, for example crates.io classifies Regorus' license as "non-standard":

     $ curl -s https://crates.io/api/v1/crates/regorus/0.4.0 | jq .version.license
     "non-standard"

Using `license` would provide better compatability with various tools.

Signed-off-by: Burak Varlı <burakvar@amazon.co.uk>

Author: unexge

Cargo.toml

@@ -9,7 +9,7 @@ name = "regorus"
 description = "A fast, lightweight Rego (OPA policy language) interpreter"
 version = "0.5.0"
 edition = "2021"
-license-file = "LICENSE"
+license = "MIT"
 repository = "https://github.com/microsoft/regorus"
 keywords = ["interpreter", "no_std", "opa", "policy-as-code", "rego"]