[BEAM-13988] Update mtime to use time.UnixMilli() calls

[jrmccluskey]

Removes the last manual calculations for millisecond time, routes calls through time.UnixMilli() instead now that the Go SDK is using Go 1.18

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Codecov Report

Merging #17578 (121cc19) into master (a0684f5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #17578   +/-   ##
=======================================
  Coverage   73.93%   73.94%           
=======================================
  Files         691      691           
  Lines       91562    91560    -2     
=======================================
+ Hits        67700    67704    +4     
+ Misses      22629    22623    -6     
  Partials     1233     1233           

Flag	Coverage Δ
go	50.21% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdks/go/pkg/beam/core/graph/mtime/time.go	68.42% <100.00%> (+13.42%)	⬆️

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[lostluck]

LGTM Thanks!