build: bump to agave 2.0.8

• remove slot_epoch mock from svm-executor

Problem:
slot_epoch was a trait function, but never used anywhere.
Igloo will also implement Automated Block Height Management.

ref:
anza-xyz/agave@106d4cf

pub trait ForkGraph {
    /// Returns the BlockRelation of A to B
    fn relationship(&self, a: Slot, b: Slot) -> BlockRelation;

    /// Returns the epoch of the given slot
    //fn slot_epoch(&self, _slot: Slot) -> Option<Epoch> {
    //   Some(0)
    //}
}