LIBDRUM-790. Updated "README-DRUM.md" for DSpace 7

[dsteelma-umd]

Updated the "README-DRUM.md" for DSpace 7:

• Removed links to documents that are no longer included in the repository
• Rewrote "Building Images for K8s Deployment" instructions to use Kubernetes "build" namespace. This is needed because "arm64"-based Apple laptops can not build the "amd64" images used by Kubernetes

https://umd-dit.atlassian.net/browse/LIBDRUM-790

References

Add references/links to any related issues or PRs. These may include:

• Fixes #[issue-number]
• Related to REST Contract

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