Move the improvements section as h2

[codebien]

What?

UX section as h2 in release notes

Why?

#3360 (comment) and it is how we did in the previous versions https://github.com/grafana/k6/blob/master/release%20notes/v0.46.0.md#ux-improvements-and-enhancements

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (8457c91) 72.95% compared to head (e48dcb6) 72.95%.

❗ Current head e48dcb6 differs from pull request most recent head 93f3783. Consider uploading reports for the commit 93f3783 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3368   +/-   ##
=======================================
  Coverage   72.95%   72.95%           
=======================================
  Files         261      261           
  Lines       20018    20018           
=======================================
  Hits        14605    14605           
  Misses       4486     4486           
  Partials      927      927           

Flag	Coverage Δ
ubuntu	72.90% <ø> (ø)
windows	72.81% <ø> (+<0.01%)	⬆️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.