fix: swap MD5 with SHA256 for TLS to work with FIPS operator image

[tip226]

Summary:

This PR updates the MongoDB Kubernetes Operator to support FIPS-compliant environments by replacing the MD5 hash algorithm with SHA-256 during SCRAM credential generation.

The motivation behind this change is a runtime panic triggered by OpenSSL when MD5 is used in environments with FIPS mode enabled. The crash occurs specifically during SCRAM credential creation, with errors like:

panic: EVP_DigestInit_ex
openssl error(s):
error:0308010C:digital envelope routines::unsupported
error:03000086:digital envelope routines::initialization error

goroutine 242 [running]:
...
github.com/mongodb/mongodb-kubernetes-operator/pkg/authentication/scramcredentials.md5Hex

To address this, the patch swaps MD5 with SHA-256, a FIPS-approved algorithm that works seamlessly in the same context.

I validated this by deploying a TLS-secured MongoDBCommunity ReplicaSet using a patched FIPS-compatible MongoDB image and a Helm-based setup that includes cert-manager for issuing certs. The change allows the operator to successfully create SCRAM credentials without triggering FIPS-related crashes.

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[tip226]

Closes #1709

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