Move long[] group varint to backward-codecs

[jpountz]

Description

This is a follow-up of #15104. The benchmark was accidentally calling legacy code that writes the output of a varint group into a long[]. This wouldn't have happened if this code - which is only used by backward codecs - was in lucene/backward-codecs.

[Exporterhe]

Hi, I'm new to open source and would like to start contributing. I'd like to work on this issue — could you please assign it to me or guide me on how to get started?

[jpountz]

@Exporterhe I'd suggest for #15116 to be complete before starting working on this. FWIW I prefer to not assign issues, just open a PR when you have time and link this issue? The odds that someone else is working on the same issue at the same time are very low.

[jpountz]

@Exporterhe You can look into it now.

[SAKSHIC]

@jpountz, I have raised the PR #15186
Please review. I will respond to the review comments as quickly as I can.

[SAKSHIC]

@jpountz Gentle reminder to review #15186