doc: update kernel and glibc reqs for PPCle

We've moved master to build on centos7 for PPCle, adjust
the glibc and kernel levels accordingly.

PR-URL: https://github.com/nodejs/node/pull/28162


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>