Add description field to workspace model for human-readable workspace titles
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MR: Pending
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## Description
As part of our [new designs for Workspaces](https://gitlab.com/groups/gitlab-org/-/epics/18674), we need to add a description field to the workspace model so that users can have human-readable workspace titles instead of relying solely on workspace name which is randomly generated.
## Acceptance criteria
- [ ] Add `description` field to workspace model
- [ ] Include database migration for adding the new field
- [ ] Field should allow for reasonable length descriptive text
- [ ] Field should be optional/nullable to maintain backward compatibility
- [ ] Update workspace GraphQL type to include description field
## TODO: Implementation plan
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