Remove support of Java 8

[dadoonet]

Hey dear users

I'd like to remove support for Java 8 from the 8.x release.
Ideally I'd prefer to do this before releasing the long overdue 8.x version.

Elasticsearch 8.0 server only supports JVM from Java 17 but that said the Elasticsearch Java Client still supports Java 8.

[swallez]

FWIW the Elasticsearch Java client version 9 will use Java 17 as its baseline, following the lead of Spring version 6.

[dadoonet]

Thanks Sylvain!

[dadoonet]

Based on @swallez's comment above, I think we should keep Java 8 for the 8.x branch and when 8.17 will be released, move the main branch to ES 9.x and Java 17.