Follow-up on #20296, #20311, #20301.
Updating InteractiveLPBackend to:

• use the standard-form transformation,
• simplify its code slightly by making use of the new objective_constant_term handling in InteractiveLPProblem
• make the example of optimizing over the dodecahedron more natural and remove use of backend methods.
• change default base_ring to QQ -- a much saner default because it's so much faster

Depends on #20296
Depends on #20311
Depends on #20301

CC: @novoselt @dimpase @videlec

Component: numerical

Author: Matthias Koeppe

Branch/Commit: c8fa4b0

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/20413