Remove unnecessary major.minor version from gitian-descriptors

Summary:
Currently this version number impacts our release process, but updating/maintaining it is unnecessary.
The package name specified in gitian-descriptors affects the outputs of the following files while gitian building:

• *-res.yml The manifest file. We currently include this in releases.
• *-desc.yml An intermediate file used to build the manifest.
• *-build.assert Used to sign builds. This file is not currently part of our process but may be in the future.

All of these files are located in versioned directories and/or contain complete version numbers within,
so maintaining a version number (partial or not) in the gitian-descriptors package names gives us little benefit.

Test Plan: Run gitian builds on CI and check that *-res.yml files no longer contain partial version numbers.

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4889