Add gen cmd and lib deprecations (SYN-9906)

[mikemoritz]

Companion to #4637

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.65%. Comparing base (dae0428) to head (452743a).

Additional details and impacted files

@@                     Coverage Diff                      @@
##           synapse-2xx-deprecations    #4638      +/-   ##
============================================================
- Coverage                     97.66%   97.65%   -0.01%     
============================================================
  Files                           295      295              
  Lines                         62032    62032              
============================================================
- Hits                          60581    60579       -2     
- Misses                         1451     1453       +2     

Flag	Coverage Δ
linux	97.65% <ø> (-0.01%)	⬇️

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