chore(dependabot): increase PR limit

[andrewazores]

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Related to #1695

Description of the change:

Increases the Dependabot open PR limit from 10 to 20

Motivation for the change:

This project has quite a few dependencies, and we often get stuck waiting for downstream versions of dependencies to be available before merging Dependabot PRs. Meanwhile, other dependencies get updated but we miss the notifications because we already have 10 pending PRs open and waiting. I think it's worth testing increasing the limit so we don't let other things, which may not necessarily require a downstream equivalent (ex. Maven plugins), get too out of date.

[github-actions]

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Required labels are : chore,ci,cleanup,docs,feat,fix,perf,refactor,style,test