[Change Proposal] Make version optional in package manifests

### Proposal

Make `version` optional in package manifests.

### Why?

In the vast majority of cases (if not all) `version` is redundant with the latest version in the changelog. Package developers are forced to manually keep these values coherent.
This has been reported as a common papercut for contributions in packages, specially for external contributors.

### How?

We would make `version` optional in package manifest for backwards compatibility, and maybe remove it in a following major. In the cases where the `version` is not set in the manifest, the version of the package would be just the version at the top of the changelog (assuming it is correctly ordered).
`elastic-package` could maybe add the version to the manifest in built packages in case it helps on package publication, discovery and so on.

We need to assess the changes needed in package spec, elastic package, package registry and fleet.

Take also into account the use cases of the `-next` suffix.

### Related issues

* https://github.com/elastic/package-spec/issues/140