Further define size of member projects for Open Source Partners program

Why is this change being made?

Based on feedback from @c_hupy and @dplanella, it'd help to define with a "large" or "prominent" open source project is. Christina also brought up the point that we may want to reassess using the word "prominent" as it implies a hierarchy.

For now, I've added a definition of at least 10,000 community members or users to qualify for our program. This may still be too small.

Some considerations:

• Some open source projects do not measure their reach due to privacy concerns around metrics and the logistics around measuring size
• What do we count as "community members"? Are these contributors + consumers? If we say we are basing it on contributors, how do we define that? e.g. GNOME's core contributor base is on the scale of a couple hundred, but they have thousands of contributions, and their user base is much larger (millions).

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