[CIVIS-9192] ENH clean up database credential IDs at civis.APIClient

[jacksonlee-civis]

This pull request tidies up the methods and properties of a civis.APIClient instance related to database credential IDs:

• Deprecate the method get_database_credential_id, with no replacement planned. This method is no longer compatible with the current state of Civis Platform.
• Add the new property default_database_credential_id, which is effectively an alias to the current less-clearly named property default_credential.
• Deprecate the property default_credential in favor of the new default_database_credential_id.

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• (For Civis employees only) Reference to a relevant ticket in the pull request title
• Changelog entry added to CHANGELOG.md at the repo's root level
• Description of change in the pull request description
• If applicable, unit tests have been added and/or updated
• The CircleCI builds have all passed

[leanne73]

One question. Generally this looks great! Thanks for taking on this cleanup.

[leanne73]

LGTM!