Update Elixir used in CI to 1.8.1

[arkgil]

No description provided.

[codecov-io]

Codecov Report

Merging #23 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #23   +/-   ##
=======================================
  Coverage   93.44%   93.44%           
=======================================
  Files           1        1           
  Lines          61       61           
=======================================
  Hits           57       57           
  Misses          4        4

Flag	Coverage Δ
#1_5_otp18	93.44% <ø> (ø)	⬆️
#1_5_otp20	93.44% <ø> (ø)	⬆️
#1_8_otp19	93.44% <ø> (ø)	⬆️
#1_8_otp21	93.44% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
lib/telemetry_metrics.ex	93.44% <0%> (ø)	⬆️

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