Minimum Supported Swift Version

[chinedufn]

We should design and document a Minimum Supported Swift Version policy.

The original motivation for this is that a MSSV may allow us to take advantage of new Swift features (such as typed throws and noncopyable generics) without needing to bump to a new swift-bridge major version.

Avoiding major version bumps will allow users to continue to receive semver compatible swift-bridge updates (performance optimizations, new APIs, etc) without needing to first notice that there is a new major version of swift-bridge available.

One trade-off of this is that users that are on older versions of the Swift compiler would need to either upgrade their Swift compiler or pin swift-bridge to a specific version.

One potential idea would be to start with an aggressive MSSV, then relax it if we find that it causes issues for people.