[DOCS] Note HTTP proxy settings for GCS snapshot repo are only available in 8.0+

[jrodewig]

#82737 adds HTTP proxy settings for GCS snapshot repositories to 8.0+. Given that the S3 and Azure repositories support similar settings, we should consider updating the 6.8 and 7.17 docs for the GCS repo type to note these settings are available in 8.0+.

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[jrodewig]

Closed by #84004