Update the dependency version floors under the agreed criterion #7644

TRL's dependency floors predate the criterion agreed in #7116 and written up in #7638. Several are older than the rule allows, and two dependencies had no floor at all.

The rule: a floor older than 2 years can be dropped freely; under that we weigh how hard the old version is to keep against who still depends on it; and we never require a release younger than 1 year. vLLM has its own 5-month window, rolling, so it is maintenance rather than a task here.

Out of scope: transformers stays at 4.56.2, since it is a core dependency that everyone is on and the wide supported range, with the branching it costs, is deliberate. datasets 4.7.0 is younger than 1 year, which #7116 already noted; the rule governs new bumps, so there is nothing to undo.

Progress

• Require peft>=0.17.0 and drop the target_parameters guards #7640
• Declare a minimum bitsandbytes version #7641
• Require torch>=2.6.0 and drop the pre-2.6 FSDP2 and accelerator fallbacks #7642
• Require accelerate>=1.10.1 and drop the ParallelismConfig shim #7643
• Require accelerate>=1.11.0 and drop the FSDP2 ignored-modules fallback #7797
• peft to 0.18.0, eligible 2026-11-13. I will address it.
• accelerate to 1.12.0, eligible 2026-11-21, for the three remaining guards. I will address it.