fix: add 8-core minimum for codespaces

[robrap]

• I've been told that a 4-core codespace isn't powerful enough to get started.
• Our doc https://2u-internal.atlassian.net/wiki/spaces/ENG/pages/2003370007/Devstack+on+Codespaces+Guide recommends setting up with 8-core.

This change simply makes this the minimum requirement.

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I've completed each of the following or determined they are not applicable:

• Made a plan to communicate any major developer interface changes (or N/A)

[Copilot]

Pull Request Overview

This PR adds a minimum CPU requirement to the development container configuration to ensure adequate performance for development environments.

• Adds an 8-core CPU minimum requirement to the devcontainer configuration

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