Generate MSRV in Kotlin file (#3869)

Currently, when generating Rust crates, the MSRV is read from
`gradle.properties` by determining the project root using `git
rev-parse`. However, some build environments smithy-rs needs to build on
don't have `git` available.

This commit uses Gradle's `rootDir` to determine the project root, and
generates a Kotlin file exposing the MSRV as part of a Gradle build task
that is then published as part of `codegen-core.jar`. This also has the
benefit that consumers of smithy-rs can read the MSRV from there.

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