Only combine PRs if more than one exists

[gedwards1]

This PR changes the combine-prs logic to only open a combine pull requests if more than one matches the provided criteria.

Why
I believe the intention of this Action is to provide a more efficient way to manage multiple PRs at once. When there's only one PR open, opening a 'combined' pull request with only that PR doesn't improve efficiency, as it's effectively just a duplicate of the existing PR.

Considerations
Should we add a new flag that allows the user to control "combine" a single PR?

• The only use case I could think of is where someone has a workflow that exclusively works uses the PR created by this action rather than the dependabot PRs. This seems so too niche to put in the work for such a flag.
• If we had flag and defaulted it to the action's existing behavior then this PR would no longer have breaking changes. That's the best reason I can think of to add a flag.

[gedwards1]

I'd appreciate feedback on my change. This PR contains a breaking change and I don't want to provide a bad experience to the downstream users. Also this is my first time contributing to open source code, so criticism is very welcome!

[GrantBirki]

@gedwards1 Thank you for this PR! I think this will be rather helpful for folks and remove duplicate work through PRs with only a single PR combined into it.

I really liked your suggestion to having a new "input flag" that gives the user control of the threshold of how many PRs should exist before they can be combined together. I went ahead and pushed this commit which does exactly that -> 633255c