CI: Deprecation of AMD SEV legacy support on CoCo Operator

[AdithyaKrishnan]

As a result of many users transitioning from SEV to SNP, we are officially deprecating and removing SEV support from CoCo Operator.
For more context, please refer to the announcement and discussion thread on the CNCF confidential-containers Slack channel: Link.

This issue was raised as an action item during the CoCo CI Working Group meeting on February 5th, 2025.
For additional details, refer to the meeting minutes: Link.

cc: @wainersm @stevenhorsman @fidencio @fitzthum @sprt @ldoktor @ryansavino @npmccallum