Add Note About Changing The Certificate Hashing Algorithm in Backend JWT Generation

[hisanhunais]

Description:
With [1] and [2], a config was provided to use SHA256 hashing algorithm for certificate hashing in backend JWT generation as follows. Need to add a note about this in [3]

deployment.toml

[apim.jwt]
enable = true
use_sha256_hash = true

[1] wso2/carbon-apimgt#12365
[2] wso2/carbon-apimgt#12387
[3] https://apim.docs.wso2.com/en/4.3.0/deploy-and-publish/deploy-on-gateway/api-gateway/passing-enduser-attributes-to-the-backend-via-api-gateway/