Fix Azure login method and utilize DefaultAzureCredential

[starcraft66]

Description

This PR fixes a logic bug that made authenticating to Vault via the azure login method never actually work unless a JWT was pre-supplied because of a logic error. Additionally, more logic errors due to copy/paste typos were fixed and the azure authentication method was changed from ManagedIdentityCredential to DefaultAzureCredential (which includes ManagedIdentityCredential, but is more flexible as it also includes more authentication options which cover more use-cases), implications of this can be found here: https://learn.microsoft.com/en-gb/azure/developer/go/sdk/authentication/credential-chains#defaultazurecredential-overview

Relates OR Closes #0000

Checklist

• Added CHANGELOG entry (only for user-facing changes)
• Acceptance tests where run against all supported Vault Versions

Output from acceptance testing:

I did not run any acceptance tests as it is not clear how to run them or what the requirements are from this PR template, I just made sure the unit tests still pass and I can confirm that this PR is working fine as I have been using this branch in production for the last 6 months.

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