Add compatibility with Ruby 3 for 6.x

[dersnek]

This is the same change made in https://github.com/elastic/elasticsearch-rails/pull/992, but for version 6.x, to enable support for Ruby 3

All credit for this fix goes to @indirect, I just copy-pasted that code over.

[cla-checker-service[bot]]

💚 CLA has been signed

[dersnek]

I've read and signed the contributor agreement, but it's not being updated.

Edit -- it was updated after my comment

[olimart]

hello @picandocodigo Is it reasonable to expect the 6.x branch to support Ruby 3?

note for later: if merged, need to update CI matrix.