Enable stable & unstable versions in release workflow (#3201)

## Motivation and Context
Addresses `TODO(GA)` from
https://github.com/smithy-lang/smithy-rs/pull/3082

## Description
This small PR will remove a guard so we can truly specify different
versions for stable crates and unstable crates. Previously, whatever
that's entered in `Stable semantic version` in the release workflow
textbox (see a screenshot in the above PR) is overwritten by what's
entered in `Unstable semantic version`. With this PR, whatever that's
entered in `Stable semantic version` will make its way to
`smithy.rs.runtime.crate.stable.version` in `gradle.properties`.

In addition, with this PR, the name of a release branch is derived from
what we enter in `Stable semantic version` in the release workflow
textbox.

## Testing
~~It has been tested in the above PR.~~

EDIT:
Found a bug I introduced in the above PR where `publisher` was not
reading a metadata `package.metadata.smithy-rs-release-tooling`
correctly in a manifest file. Fixed it in c477d2f, confirmed that a
[dry-run](https://github.com/smithy-lang/smithy-rs/actions/runs/6872502257)
passed, and verified the release artifact contained both stable crate
versions & unstable versions as expected.

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