Simplify release process for Swift build

[simolus3]

This removes the kmmbridge invocation when publishing our SDK. Instead, this moves towards a model where we tag our releases and create GitHub releases for them. An action will compile our XCFramework and attach it to these releases, which we can then use as a binaryTarget() source in our Swift SDK.

This means that:

1. We stop publishing a SwiftPM build from here - users no longer need to close this repository as part of their Swift build.
2. We no longer upload XCFrameworks to maven central - only the Kotlin SDK itself will be uploaded going forward.
3. Our Swift SDK will always use a fixed download URL, there's no longer a way to update the Kotlin SDK and those changes reaching downstream Swift users without us updating the Swift SDK too.

I have updated the documentation on the release process here. For testing purposes, I copied the steps creating the release into a temporary GH action. We can use that temporary release as 1.1.0 (since the Kotlin SDK has already been published) and then use the automation for subsequent releases.

The Swift SDK integrates these changes in: powersync-ja/powersync-swift#46