Update pod-identities.md

[Ramneek-kalra]

Summary: Updating Pod Identity Considerations in case when customer might be using "Security Group of Pods" as well alongside Agent.

Issue #, if available: Just an additional consideration when customer might be using "Security Group for Pods" along with Pod Identity Agent. This is to ensure that there's no breakage in the Agent working (As also found via replication).

Description of changes: Added one additional consideration as below:

• If you're using Security Group of Pods along with Pod Identity Agent, please have "POD_SECURITY_GROUP_ENFORCING_MODE" Flag in VPC CNI as per considerations stated at: Security Group of Pods.

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