[r] Release 1.4.0

[aaronwolen]

Issue and/or context: #1567

Changes: Bumps the version to 1.4.0 in preparation for release.

Notes for Reviewer: Also deemphasizes main-old in the README to keep the focus here.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -11.47% ⚠️

Comparison is base (a103172) 65.05% compared to head (eafa617) 53.58%.
Report is 1 commits behind head on main.

❗ Current head eafa617 differs from pull request most recent head 37ba42b. Consider uploading reports for the commit 37ba42b to get more accurate results

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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1574       +/-   ##
===========================================
- Coverage   65.05%   53.58%   -11.47%     
===========================================
  Files         105       74       -31     
  Lines        8802     6134     -2668     
===========================================
- Hits         5726     3287     -2439     
+ Misses       3076     2847      -229     

Flag	Coverage Δ
python	?
r	53.58% <ø> (ø)

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see 31 files with indirect coverage changes

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[eddelbuettel]

Tadaa -- here we go!