[ Deprecation ] Add logic to handle deprecation of root-using FE

[gwright99]

Background

Release 1.7.0 (supporting Platform v25.3.0) was already changed to use the unprivileged FE. The root image remains available, however, and could be swapped in.

Current Seqera DevOps planning calls for the following deprecation flow (internal link). :

• Root image (currently unqualified) will get a qualification tag in Platform release v26.1
• Root image to no longer be published starting with Platform v26.2 This should be handled via validation.

Tasks

• Add checks to handle existence of root image with new tag in Platform v26.1.
• Add checks to handle lack of existence of root image if Platform is v26.2.