SGX removal from main

[achamayou]

As per 5.0 release notes, release/5.x is the last LTS with SGX support. 6.0 (main at the moment) will target confidential containers exclusively, and switch from Ubuntu to Azure Linux 3.0.

This issue tracks the various steps involved in the removal of SGX from main, the list is a work-in-progress:

• [x] Remove SGX attestation verifcation/oehostverify dependency from virtual and snp builds (#6423)
• [x] Remove SGX target (but keep build-* image creation for SGX to support 5.x/4.x) (#6426)
• [x] Remove CCF_DISABLE_VERBOSE_LOGGING (#6426)
• [x] Remove SGX from release playbooks, but keep it in ccf-dev for build-* images (#6447)
• [ ] Migrate code upgrade tests from SGX to Virtual
• [ ] Merge SNP and Virtual targets, SNP presence is detected at runtime
• [ ] Remove INSIDE_ENCLAVE

[achamayou]

Goal: Build, tests and release artefacts are completely free from SGX dependencies.

This does not include undoing or revisiting the architectural impact of SGX (e.g. the ringbuffer), which will be a separate issue/collection of PRs. Other follow-ups include the migration to Azure Linux, and upgrade of the host-side code to OpenSSL 3.x.