It's available in RubyGems since 3.2.3 and we no longer support that old
version.

https://github.com/rubygems/rubygems/commit/71bcf354f5

https://github.com/rubygems/rubygems/commit/85371a961a

String#split supports a block since Ruby 2.6, avoiding intermediate array.

https://github.com/rubygems/rubygems/commit/4e864a8f2e

This changes the CompactIndexClient to store etags received from the
compact index in separate files rather than relying on the MD5 checksum
of the file as the etag.

Smoothes the upgrade from md5 etags to opaque by generating them when no
etag file exists. This should reduce the initial impact of changing the
caching behavior by reducing cache misses when the MD5 etag is the same.

Eventually, the MD5 behavior should be retired and the etag should be
considered completely opaque with no assumption that MD5 would match.

This still allocates a ton (a string for each line, plus a bunch of
splits into arrays), but it helps a bit when Bundler has to go through
dependency resolution.

```
==> memprof.after.txt <==
Total allocated: 194.14 MB (2317172 objects)
Total retained:  60.81 MB (593164 objects)

==> memprof.before.txt <==
Total allocated: 211.97 MB (2404890 objects)
Total retained:  62.85 MB (640342 objects)
```

https://github.com/rubygems/rubygems/commit/c68b41b0e5

  from https://github.com/rubygems/rubygems/commit/bfb0ae69776069155d2092702bfbb5a12617d85a

  Picked at 12aeef6ba9a3be0022be9934c1a3e4c46a03ed3a