DOC-5707, DOC-5708 improve attributes for DITA

[aimurphy]

• Change db-classic attribute to follow product name and terminology in IBM docs (Astra Managed Clusters deployments). (Yes, I know, it's awkward 😬)
• Change one instance of {scb-brief} to {scb}, and then remove {scb-brief} from antora.yml. This attribute should only be used for UI labels, otherwise use either {scb} or {scb-short}.
• Rephrase one line to avoid a pluralized attribute.

[plpesvc-ds]

Build successful! ✅
Pull requests with matching branch name doc-5707 included in the build:

• DOC-5707, DOC-5708 improve attributes for DITA #40 (this pull request)
• https://github.com/riptano/docs-home/pull/221
• https://github.com/riptano/astra-vector-docs/pull/1405
• https://github.com/riptano/cql-docs/pull/147
• https://github.com/riptano/dev-app-drivers/pull/66

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