Add master as trigger for Integration-Checks

[dcollins15]

Picking up where #9538 left off, this PR:

1. Drops the AppVeyor CI pipeline by removing the appveyor.yml file, per my the plan laid out here
2. Adds master as a push & PR trigger for the Integration-Checks workflow. Eventually, we'll be transitioning the repo over to using main and I wasn't initially sure if that would happen before or after the upcoming release, but now I'm sure it'll be in the new year.

[rharao]

Are there plans for a linter?

[rharao]

By the way, the Appveyor badge can be removed from the README

[dcollins15]

Are there plans for a linter?

Linting is very high personal wish list but I'm hesitant to roll it out until we're able to get our test coverage up—with the current state of things, I'd be worried applying a tool like lintr could quietly introduce regressions into some of our messier code. We could probably start rolling out linting over time to be more conservative but I honestly haven't thought very much about how we'd go about it.

Is there a particular tool you'd recommend?

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I thought the AppVeyor workflow would stop running after I removed the relevant webhook in the repository settings but it seems to still be getting triggered 😫 It seems like a common issue with an apparently simple solution but I'm running short on time for trouble shooting so I may just restore appveyor.yaml again and circle back to this later.