[Maikol]

This moves EpochManager and its storage contract from packages/contracts to graph-core with identical bytecode, and moves horizon's EpochManager tests with them. EpochManager's SafeMath import now goes through @openzeppelin/contracts-v3, an alias of OpenZeppelin 3.4.2, because graph-core uses OpenZeppelin 5, and a new lint step fails if any other package imports that alias.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.74%. Comparing base (a4c3c06) to head (509197e).
⚠️ Report is 2 commits behind head on chore/contracts-removal-phase-0.

Additional details and impacted files

@@                       Coverage Diff                        @@
##           chore/contracts-removal-phase-0    #1398   +/-   ##
================================================================
  Coverage                            98.74%   98.74%           
================================================================
  Files                                   39       39           
  Lines                                 2699     2699           
  Branches                               477      477           
================================================================
  Hits                                  2665     2665           
  Misses                                  13       13           
  Partials                                21       21           

Flag	Coverage Δ
unittests	98.74% <ø> (ø)

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