Fixing Bicep deploy-time warnings

[BrandonSharp]

Description

Syntax changes to address deploy-time warnings.

use-resource-id-functions - fixed by referencing the built-in policy defs using tenantResourceId() as described here: https://docs.microsoft.com/en-us/azure/azure-resource-manager/bicep/bicep-functions-resource#tenantresourceid

prefer-unquoted-property-names - removed single quotes from keys in the defaultTags variable in mlz.bicep

no-unused-existing-resources - fixed by removing unused securityContacts resource from log-analytics-diagnostic-logging.bicep

use-stable-resource-identifiers - This warning was fixed as a result of prior work done in PR #748

Remaining changes are VSCode formatting artifacts, which I can revert if necessary/desired.

Issue reference

The issue this PR will close: #743

Checklist

Please make sure you've completed the relevant tasks for this PR out of the following list:

• All acceptance criteria in the backlog item are met
• The documentation is updated to cover any new or changed features
• Manual tests have passed
• Relevant issues are linked to this PR