rename GoodContrib and GoodFirstBug to match new labels promoted by Github

[derekbruening]

Github is suggesting we use certain labels:

Now, GitHub will help potential first-time contributors discover issues
labeled with help wanted or good first issue

That would mean renaming our existing GoodContrib to help wanted and GoodFirstBug to good first issue and updating references to the label names in the wiki.

Other projects have asked (e.g., nunit/governance#31 (comment)) and apparently custom names for the same purpose is not supported. While it is a little ugly to not match the CamelCase of the other labels, it may be worth the rename, depending on what exactly Github is doing with these.

[derekbruening]

I went ahead and did this for both DR and Dr. Memory and updated the wiki pages.