Deprecate migrator package

[Fs02]

• Extract migrator package to https://github.com/go-rel/migration to prevent cyclic reltest dependency
• Existing migrator package is marked as deprecated

[codecov]

Codecov Report

Merging #267 (e125764) into master (f772d9c) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #267   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           33        32    -1     
  Lines         2790      2723   -67     
=========================================
- Hits          2790      2723   -67     

Impacted Files	Coverage Δ
cmd/rel/internal/migrate.go	100.00% <ø> (ø)
migrator/migrator.go

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