Update to go1.21.3 and use log/slog instead of slog-shim

[Brandon-M-Hartman]

Simple changes, just moving up to go1.21.3 and log/slog instead of using the slog-shim that was used previously. Also removed the deprecated Logger interface as nothing in the code uses it anymore.

[CLAassistant]

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[github-actions]

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[sagikazarmark]

@Brandon-M-Hartman could you please rebase your branch to the latest master? Thanks!

[sagikazarmark]

Nevermind, I ended up doing it myself. Thanks for the contribution!