Implement _get_lastbday

[jbrockmendel]

Orthogonal to other PRs.

Implement _get_lastbday mirroring _get_firstbday.

Add a dummy argument to get_firstbday so that the signatures match. Following this and #18218, we'll be able to define e.g.

class BQuarterBegin(...):
    _get_bday = _get_firstbday

class BMonthEnd(...):
    _get_bday = _get_lastbday

etc. From there we can get rid of bunch of duplicated logic.

[codecov]

Codecov Report

Merging #18234 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #18234      +/-   ##
==========================================
- Coverage   91.43%   91.39%   -0.05%     
==========================================
  Files         163      163              
  Lines       50091    50092       +1     
==========================================
- Hits        45800    45780      -20     
- Misses       4291     4312      +21

Flag	Coverage Δ
#multiple	89.2% <100%> (-0.03%)	⬇️
#single	40.36% <7.14%> (-0.06%)	⬇️

Impacted Files	Coverage Δ
pandas/tseries/offsets.py	97.11% <100%> (ø)	⬆️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/plotting/_converter.py	63.38% <0%> (-1.82%)	⬇️
pandas/core/frame.py	97.8% <0%> (-0.1%)	⬇️

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[jbrockmendel]

Too many open PRs, need to exercise restraint. Closing.