Migrate @material-ui to @mui and enable the Backstage proxy to communicate with the OpenCost service

[and-arevalo]

Hey, I just made a Pull Request!

This PR addresses the following issues:

• Resolved OpenCost Plugin Breakage:

  • The OpenCost plugin was experiencing issues due to changes in its dependencies.
  • This PR includes necessary fixes to ensure the plugin functions correctly.

• Migrated from @material-ui to @mui:

  • Upgraded to the latest MUI library for access to the latest features.

• Enabled Backstage Proxy for OpenCost Service Communication:

  • Configured the Backstage proxy to facilitate communication with the OpenCost service. This enables users to integrate Backstage with private Opencost installations.

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-opencost	workspaces/opencost/plugins/opencost	minor	v0.3.0

[mattray]

It looks OK to me, but I'm not able to test it right now. If someone could validate it works for them I'm happy to approve it.

I'm no longer involved with OpenCost, so I'm more than happy to hand off responsibility to someone else.

[vinzscam]

Unfortunately migrating to material-ui v6 is not an option (see backstage/backstage#27726) 😞