fix gr legend

[t-bltg]

Fix #4491.
Pin GR_jll per https://discourse.julialang.org/t/plots-jl-gr-jl-broken-due-to-gr-jll-v0-69-1-1/89708/2.

EDIT: yanking GR_jll instead (JuliaRegistries/General#71554).

[codecov]

Codecov Report

Base: 90.89% // Head: 90.89% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (2e04cb7) compared to base (6206ac0).
Patch coverage: 90.47% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4492      +/-   ##
==========================================
- Coverage   90.89%   90.89%   -0.01%     
==========================================
  Files          40       40              
  Lines        7647     7641       -6     
==========================================
- Hits         6951     6945       -6     
  Misses        696      696              

Impacted Files	Coverage Δ
src/backends/gr.jl	92.59% <85.71%> (ø)
src/axes.jl	90.15% <87.50%> (ø)
src/layouts.jl	95.93% <100.00%> (-0.07%)	⬇️

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