Remove outdated Supporting sections

[jeremyevans]

What was the end-user or developer problem that led to this PR?

I noticed out of date documentation in Supporting sections. These sections made sense when Ruby Central was managing rubygems and bundler, but now that they are managed by Ruby core, they are no longer accurate.

What is your fix for the problem, implemented in this PR?

This removes the sections. While we could replace the wording and links, other repositories managed by Ruby core generally don't have similar wording, so I think it's best to remove it.

Make sure the following tasks are checked

• Describe the problem / feature
• Update documentation to solve the problem

[colby-swandale]

👋🏻 I have raised this internally and the feedback has been yes the paragraph should be tweaked, but shouldn't be removed entirely. I hopefully should have a response back in the next few days for new copy.

[schneems]

I think this is a reasonable assertion of where we're at:

## Supporting

RubyGems is a community project, please consider sponsoring [individual contributors for their great OSS 
work](https://github.com/ruby/rubygems/graphs/contributors).

In addition, Ruby Central administers grant-funded work for improvements to `ruby/rubygems`, as well as running
 RubyGems.org (the service). You can support Ruby Central by attending or [sponsoring]([email protected]) 
 a [RubyConf](https://rubyconf.org/), or by [joining as a supporting member](https://rubycentral.org/#/portal/signup).

It does a few things:

• Puts the emphasis on the individuals making an impact.
• Distinguishes that this is a community project. (You do not need to be a part of RC to help; contributions welcome)
• Acknowledges Ruby Central is involved in helping to fund work. But the wording (feedback welcome) helps to distinguish that Ruby Central does not "run" or "own" this project.

Edit: Adding a link to https://www.ruby-lang.org/en/news/2025/10/17/rubygems-repository-transition/.