DOC: replaced 'Good as first PR' with 'good first issue'

[raybellwaves]

• [NA] closes #xxxx
• [NA] tests added / passed
• [NA] passes git diff upstream/master -u -- "*.py" | flake8 --diff
• [NA] whatsnew entry

Was browsing https://github.com/pandas-dev/pandas/tree/master/doc and it seemed the 'Good as first PR' had no open issues, whereas 'good first issue' has quite a few open issues.
I'm guessing the 'Good as first PR' label was not well used/got renamed to 'good first issue' label?

[codecov]

Codecov Report

Merging #18806 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18806      +/-   ##
==========================================
- Coverage   91.64%    91.6%   -0.05%     
==========================================
  Files         154      154              
  Lines       51430    51430              
==========================================
- Hits        47131    47110      -21     
- Misses       4299     4320      +21

Flag	Coverage Δ
#multiple	89.46% <ø> (-0.03%)	⬇️
#single	40.83% <ø> (-0.12%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/plotting/_converter.py	64.78% <0%> (-1.74%)	⬇️
pandas/core/frame.py	97.68% <0%> (-0.11%)	⬇️

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[gfyoung]

Good catch!

[jreback]

thanks @raybellwaves