Redeployment with new URL secret causes failure

[jharrell-illinois]

Steps to reproduce:

1. deploy new instance to a Resource Group
2. change an environment variable in local.settings.json
3. re-deploy over the same Resource Group

Expected outcome:
bicep replaces WebhookUrl secret in keyvault with new secret
bicep replaces environment variables in functionapp with new values

Actual outcome:
WebhookUrl secret has a new version uploaded with the old version still remaining active
functionapp env var bound to that secret does not properly update, requests to new URL fail with http 500 (instead of 403)

Issues:

• old secret not being invalidated
• unexplained failure when invoking new URL, event doesn't reach splunk