Update references to master to point to main

[jni]

Description

Closes #3128.

The repo's default branch is already main, but now we need to clean up all the references in the code and docs that point to master instead of main.

[jni]

Note: I have not updated the default branch on the napari/docs repo because I got the following scary message:

ie we'd need to trigger a push ~simultaneously in order to not break the API docs. Since these are automatically generated and not user-facing, I propose we postpone that specific transition.

[codecov]

Codecov Report

Merging #3351 (4fa954a) into main (5f96d5d) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3351      +/-   ##
==========================================
+ Coverage   82.55%   82.56%   +0.01%     
==========================================
  Files         542      542              
  Lines       45147    45147              
==========================================
+ Hits        37269    37275       +6     
+ Misses       7878     7872       -6     

Impacted Files	Coverage Δ
napari/_vispy/vispy_surface_layer.py	86.20% <ø> (ø)
napari/benchmarks/benchmark_image_layer.py	0.00% <ø> (ø)
napari/benchmarks/benchmark_labels_layer.py	0.00% <ø> (ø)
napari/benchmarks/benchmark_points_layer.py	0.00% <ø> (ø)
napari/benchmarks/benchmark_qt_viewer.py	0.00% <ø> (ø)
napari/benchmarks/benchmark_qt_viewer_image.py	0.00% <ø> (ø)
napari/benchmarks/benchmark_qt_viewer_labels.py	0.00% <ø> (ø)
napari/benchmarks/benchmark_shapes_layer.py	0.00% <ø> (ø)
napari/benchmarks/benchmark_surface_layer.py	0.00% <ø> (ø)
napari/benchmarks/benchmark_vectors_layer.py	0.00% <ø> (ø)
... and 4 more

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[tlambert03]

merging so that we can have tests again. can follow up if we find additional stuff