[Bug] Update Security exercise - out of date content

**Description**
In the 'security section, 'Managed Identities' the method to retrieve the secret using a policy is out of date. You can use 'Named Values' of type 'Key Vault' now, so the exercise needs updating to reflect this approach.
Also, you're required to perform a role assignment on Key Vault to do this, which Contributor does not allow you to assign.

See this word doc
[API Management - Labs - security suggestion .docx](https://github.com/user-attachments/files/24638311/API.Management.-.Labs.-.security.suggestion.docx)

**Tasks**
1. Update 'managed Identity' exercise to show instructions based on using a Named Value of type 'Key Vault' instead of the policy method.
2. If the participant is expected to do all this themselves then make it clear in the pre-reqs that the security labs require User Access Admin (and a Key Vault). ALTERNATIVELY if this is being done as part of a larger group, to avoid multiple KVs being created, recommend adding instructions to the pre-req that specify to 1) create a key Vault for all to share, 2) create a user-assigned managed identity and assign it the relevant permissions to Key Vault 3) in the Security / Managed Identity exercise, when enabling the Identity of the APiM service, use the previously created User-Assigned Managed Identity .