DOCS: 4.x gRPC

[ljamen]

Problem Description

We need to update docs for 4.x

• update client.adoc for MP
• update server.adoc for MP
• update client.adoc for SE
• update server.adoc for SE

Guidelines

• Make sure any fixes from SE 3.x are forward ported to 4.x DOCS: SE Diffs 3.x to Main *.adoc  #7515
• Make sure any fixes from MP 3.x are forward ported to 4.x
• Initial focus is on correctness
• Record migration impact in this issue

Migration Impact

Summarize incompatibilities between Helidon 3 and Helidon 4.
Summarize actions customers must take to migrate code
OK to reference external specs or documents (for example MicroProfile specs)

[tomas-langer]

Migration impact:
Migration is currently possible only for a subset of server routes in SE. MP and client are not supported yet, some features in SE are not supported (yet). There is a follow up issue to re-introduce gRPC in full to Helidon post release.