Use "Seeking community contributions" over "Accepting merge requests"

What does this MR do and why?

We are now using the label "Seeking community contributions" to solicit community contributions, because the "Accepting merge requests" label is both vague and applied to many issues where we are not seeking community contributions. This updates all references in the documentation.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.