Replace deprecated default k6 image

[lucasmdrs]

This PR replace the deprecated k6 image default image used.

loadtest-job-0-lf2l6 k6 +------------------------------------------------------------------------------+
loadtest-job-0-lf2l6 k6 | WARNING: The loadimpact/k6 Docker image has been replaced by grafana/k6.     |
loadtest-job-0-lf2l6 k6 |          THIS IMAGE IS DEPRECATED and its support will be discontinued after |
loadtest-job-0-lf2l6 k6 |          Dec 31, 2023. Please update your scripts to use grafana/k6 to       |
loadtest-job-0-lf2l6 k6 |          continue using the latest version of k6.                            |
loadtest-job-0-lf2l6 k6 +------------------------------------------------------------------------------+

[codecov-commenter]

Codecov Report

Merging #271 (0e8b07a) into master (d18cdd3) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master     #271   +/-   ##
=======================================
  Coverage   56.16%   56.16%           
=======================================
  Files          37       37           
  Lines        4136     4136           
=======================================
  Hits         2323     2323           
  Misses       1690     1690           
  Partials      123      123           

Files Changed	Coverage Δ
pkg/backends/k6/backend.go	37.24% <ø> (ø)