Loosening lower B mass cut in B2XMuMu in S21r0p2

Loosening the cut will improve the control over the tails in the K*->K+pi0 channel. Has already been done in S28 (see !629 (diffs))

Rates stay well below 0.5% (see below)

StrippingReport INFO Event 10000, Good event 10000

| Decision name|Rate,%|Accepted| Mult|ms/evt|

|StrippingGlobal | 0.2300| 23| | 8.710|

|StrippingSequenceStreamB2XMuMu | 0.2300| 23| | 8.701|