* Merge RubyGems-3.5.6 and Bundler-2.5.6

* Merge RubyGems-3.5.7 and Bundler-2.5.7

* Merge RubyGems-3.5.8 and Bundler-2.5.8

* Partly reverted about https://github.com/rubygems/rubygems/pull/7483

* Merge RubyGems-3.5.9 and Bundler-2.5.9

https://github.com/rubygems/rubygems/commit/e015200ffa

https://github.com/rubygems/rubygems/commit/cfc5018c54

https://github.com/rubygems/rubygems/commit/0d758e8926

https://github.com/rubygems/rubygems/commit/73b9498658

https://github.com/rubygems/rubygems/commit/bb66253f2c

Additionally, the result is memoized, as it's used twice in a row.

This change does result in a net behavioral diff, as the list of ENVs
being checked has been updated (now includes buildkite, taskcluster,
cirrus, dsari, and drops buildbox and snap)

https://github.com/rubygems/rubygems/commit/3fb445a5a1

Gem::RemoteFetcher uses Gem::Request, which adds the RubyGems UA.
Gem::RemoteFetcher is used to download gems, as well as the full index.
We would like the bundler UA to be used whenever bundler is making
requests.

This PR also avoids unsafely mutating the headers hash on the shared
`Gem::RemoteFetcher.fetcher` instance, which could cause corruption or
incorrect headers when making parallel requests. Instead, we create one
remote fetcher per rubygems remote, which is similar to the connection
segregation bundler is already doing

https://github.com/rubygems/rubygems/commit/f0e8dacdec

Improve error reporting for checksums, raises a new error class.

Solve for multi-source checksum errors.

Add CHECKSUMS to tool/bundler/(dev|standard|rubocop)26_gems.rb

https://github.com/rubygems/rubygems/commit/26ceee0e76

Co-authored-by: Samuel Giddins <segiddins@segiddins.me>