chore(ci): run ubuntu packaging on 20.04 COMPASS-9917

[mabaasit]

Description

Checklist

• New tests and/or benchmarks are included
• Documentation is changed or added
• If this change updates the UI, screenshots/videos are added and a design review is requested
• I have signed the MongoDB Contributor License Agreement (https://www.mongodb.com/legal/contributor-agreement)

Motivation and Context

• Bugfix
• New feature
• Dependency update
• Misc

Open Questions

Dependents

Types of changes

• Backport Needed
• Patch (non-breaking change which fixes an issue)
• Minor (non-breaking change which adds functionality)
• Major (fix or feature that would cause existing functionality to change)

[Copilot]

Pull Request Overview

Updates the Ubuntu packaging CI configuration to run on Ubuntu 20.04 instead of 18.04, removing the glibc 2.27 compatibility requirement.

• Switches build runner from ubuntu1804-large to ubuntu2004-large
• Removes outdated glibc compatibility comment explaining the need for Ubuntu 18.04

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.evergreen/buildvariants-and-tasks.yml	Updates the run_on configuration for Ubuntu packaging
.evergreen/buildvariants-and-tasks.in.yml	Updates the source template with new Ubuntu version and removes glibc compatibility comment

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[addaleax]

Neither the PR nor the ticket describe why we're doing this, maybe we could add a bit to explain that? Also, the commit message should probably be more like feat: raise glibc requirement to 2.31, since this is a breaking platform support change