API versioning via GitHub releases/tags

Currently there's no way I can refer to a specific version of the API specifications.

This is an issue for both services, which cannot advertise the supported API specifications, and clients, who don't know if they are following the latest version because they don't know when things change. For example, see https://github.com/ethereum/execution-apis/pull/639.

One solution would be to introduce a simple semantic versioning system for the API specs.

Tagging those versions as releases/tags on the GitHub repository would also improve quality of life, helping with automation and integration.

Note: The [current tags](https://github.com/ethereum/execution-apis/tags) seem to me to be more related to the development of _possible future v1 specs_ than to real tags that developers can reference in their applications.

Note 2: There's another open issue about a similar topic (https://github.com/ethereum/execution-apis/issues/408), but I think a simple tag/release approach would be an easy first step. After that, we can consider the possibility of introducing a more complex, granular versioning system at the API level and not just the repository level.