Add support for Protobuf 6

[maroux]

Is your feature request related to a problem?

Unable to use protobuf 6 in an application that uses opentelemetry.

Describe the solution you'd like

The version pinning here ought to be updated to v6, and proto files re-compiled with protoc 6.x.

Describe alternatives you've considered

No response

Additional Context

Protobuf 6 is the actively supported version of Protobuf, proto 5 is expected to EOL this year.

Based on pypi stats, proto6 has cross proto5:

Would you like to implement a fix?

No

[emdneto]

I'll take a look on this one ✌

[emdneto]

Discussed in the SIG (Jun 19th):
Python SDK doesn’t have a strict Protobuf version policy—and since Protobuf 5.x remains supported until 2026, we have plenty of breathing room. For now, we’ve agreed to take a short-term approach: relax the opentelemetry-proto dependency as in #4620 and put #4643 on hold or perhaps work in a long-term solution based on having multiple versions of proto/exporters.