DM-32648: Change default branch to main.

[ktlim]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Merging #607 (c85bbdc) into main (7b2c2f7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #607   +/-   ##
=======================================
  Coverage   83.67%   83.67%           
=======================================
  Files         242      242           
  Lines       30703    30703           
  Branches     4595     4595           
=======================================
  Hits        25691    25691           
  Misses       3814     3814           
  Partials     1198     1198           

Impacted Files	Coverage Δ
...n/lsst/daf/butler/registry/databases/postgresql.py	77.83% <ø> (ø)

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