upgrade temporal-server to 1.15.0

[eritscher]

What changed?
Upgraded go.temporal.io/server to 1.15.0
removed schema migrations manual setup in ephemeral mode (this happens on automatically now)

[codecov-commenter]

Codecov Report

Merging #44 (9084415) into main (9561b1a) will increase coverage by 0.17%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
+ Coverage   41.75%   41.93%   +0.17%     
==========================================
  Files          11       11              
  Lines         728      725       -3     
==========================================
  Hits          304      304              
+ Misses        402      399       -3     
  Partials       22       22              

Impacted Files	Coverage Δ
server.go	59.49% <0.00%> (-0.27%)	⬇️
temporaltest/logger.go	86.66% <0.00%> (+13.33%)	⬆️

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