Move ESS data reduction requirement template from Confluence to GitHub issue template

[jokasimr]

Is this a case where we could use an ESS-specific copier template? See scipp/ess#200.

• We should consider streamlining/updating the template, based on past experience. @SimonHeybrock
• Ported to https://github.com/scipp/essdiffraction/blob/main/.github/ISSUE_TEMPLATE/high-level-requirement.yml. @SimonHeybrock
• Waiting for user feedback
• Move to a copier template.