Add dependabot configuration for default branch

[cartermckinnon]

What type of PR is this?

What this PR does / why we need it:

Adds a basic dependabot configuration for the default branch. I've limited the bot to 3 PR's at a time, and it will only run weekly. If this goes well, we can add config for the other release-* branches.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

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