Remove redundant author IDs from existing CIPs and template

## Context

> my only nit would be a question of whether or not we're going to change this on CIPs?

Yea can do. We can also update the template to include just Github username.

_Originally posted by @rootulp in https://github.com/celestiaorg/CIPs/issues/272#issuecomment-2733627752_

## Problem

Existing CIPs and the template have multiple IDs for authors.

## Proposal

1. Remove redundant author IDs from existing CIPs. Preserve Github username, remove names and emails
2. Repeat that for the CIP template

Locate the existing CIPs and the CIP template, then review how each author is identified. Update them so GitHub usernames remain while names and email addresses are removed, and verify that no redundant author IDs remain in the existing documents or template.

Written by the indexing model from the issue text.