Update flipt-grpc-go version and path

[markphelps]

flipt-grpc-go moved to https://github.com/flipt-io/flipt-grpc-go

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@b4a71f8). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #906   +/-   ##
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  Coverage        ?   81.02%           
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  Files           ?       19           
  Lines           ?     1802           
  Branches        ?        0           
=======================================
  Hits            ?     1460           
  Misses          ?      272           
  Partials        ?       70           

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