add note on SO_REUSEPORT requirement

[RafalSkolasinski]

What this PR does / why we need it:

Add note on AttributeError: module 'socket' has no attribute 'SO_REUSEPORT' to the upgrading page.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

• The issue is present on specific Python distributions: https://community.intel.com/t5/Intel-Distribution-for-Python/AttributeError-module-socket-has-no-attribute-SO-REUSEPORT/m-p/1257361/highlight/true
• PR that introduced the requirement adding multiple grpc connections for python server and executor client #3356

Difficult to establish universal minimum version requirements. For example for 3.8.x line it does not work with <3.8.5, works fine with 3.8.5 and 3.8.6. Couldn't test with 3.8.7 as there was no anaconda package, then was broken for 3.8.8 and fixed again for 3.8.10 for example. But this can actually lie in the compilation problem of the python itself so version compatibility is difficult to establish.

Does this PR introduce a user-facing change?:

[seldondev]

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