use lts keyword for CI

[t-bltg]

One should run non-regression tests:

• on the lowest declared julia version in Project.toml;
• on the current julia lts version;
• on the current julia 1 release branch.

Please run CI in the GR.jl repo when releasing a new gr version in order to avoid breaking Plots.jl, this isn't difficult.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.19%. Comparing base (29242a8) to head (448e90b).
Report is 2 commits behind head on master.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #558      +/-   ##
==========================================
+ Coverage   44.74%   47.19%   +2.44%     
==========================================
  Files           8        8              
  Lines        3057     3174     +117     
==========================================
+ Hits         1368     1498     +130     
+ Misses       1689     1676      -13     

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