Add GitHub Actions workflow to update the .NET SDK

[martincostello]

Add a GitHub Actions workflow with an action that checks for updates to the .NET SDK at 1200 UTC on Wednesdays (i.e. the day after when Update Tuesday would be).

This allows for automated PRs like this one martincostello/update-dotnet-sdk#320. As they're raised by github-actions[bot], CI builds won't be automatically triggered so you have to close and open the PR again to get the builds to run due to this restriction.

We could always set up a bot account at some point and use its PAT to raise the PRs instead to improve that (example: martincostello/sqllocaldb#568, config).

[codecov-commenter]

Codecov Report

Merging #1005 (c84043d) into main (1722749) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1005   +/-   ##
=======================================
  Coverage   71.11%   71.11%           
=======================================
  Files         137      137           
  Lines        3756     3756           
  Branches      764      764           
=======================================
  Hits         2671     2671           
  Misses        874      874           
  Partials      211      211           

Flag	Coverage Δ
linux	71.11% <ø> (ø)
macos	71.11% <ø> (ø)
windows	71.11% <ø> (ø)

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