[Fix #973] Rename sponsors section

[bbatsov]

As requested by @ReedCopsey.

[dsyme]

I feel we should look for alternative logos by asking around for companies using F# and currently willing to have logos here. The foundation material is now all at https://foundation.fsharp.org so the question of status wrt foundation isn't so important on front page.

In the interim I suggest we just remove the word "sponsors" while we get this sorted out. It's OK for this to continue to be a WIP for a bit.

[bbatsov]

So just replace "Sponsor Members" with "Members"?

[dsyme]

I guess "Sponsors Past & Present" would be ok, while we work on things

[dsyme]

(It might be we should just remove eventually and other community sites and orgs like Amplifying F# will be better placed to represent corporate interests)

[bbatsov]

@dsyme I've updated the heading as suggested by you.