Increase mesh certificate key size

[strekm]

Description

Increate certificate key size to 4096 bits by setting CITADEL_SELF_SIGNED_CA_RSA_KEY_SIZE Istio env variable.

ACs:

• keys size length is 4096 bits
• setting is documented

Reasons

DoD:

• Provide unit and integration tests.
• Provide documentation.
• Verify if the solution works for both open-source Kyma and SAP BTP, Kyma runtime.
• If you changed the resource limits, explain why it was needed.
• If the default configuration of Istio Operator has been changed, you performed a manual upgrade test to verify that the change can be rolled out correctly.
• Verify that your contributions don't decrease code coverage. If they do, explain why this is the case.
• Add release notes.

[strekm]

This change is effective for new installations. It needs to be investigated how we can enforce that for existing installations with certificates generated with weaker key. Proper planing might be needed in case operation is causing downtime.