Improve upsert throughput by 3x (#334)

## Problem

Python SDK upsert throughput is low compared to other SDKs - for example
I can achive 880 vector upserts/sec with the Python SDK, compared to
3500 upserts/sec with the Java SDK.

Profiling the Python SDK performing these upserts shows a large
percentage of time in gRPC / protobuf serialisation / deserialisation.

## Solution

Upgrade protobuf from v3 to v4. This adds a number of performance
improvements in parsing / serialization as documented at
https://protobuf.dev/news/2022-05-06/#python-updates

This increases upsert() throughput by 3x (measured by upserting 1M 768
dimension indexes to a pod-based index in batches of 500):

* Before:  880 vectors/sec
* After:  2580 vectors/sec

As per the documentation, this results in an incompatible change with
the _generated_ Python code, so this depends on a related change to
pinecone-protos to change the version of protobuf used to generate the
Python code there.

## Type of Change

- [x] None of the above: Performance improvement.

## Test Plan

Use existing regression tests.

.github/workflows/testing-dependency.yaml

@@ -56,12 +56,11 @@ jobs:
           # - 4.1.0
           - 4.3.3
         protobuf_version:
-          - 3.20.3
+          - 4.25.3
+        protoc-gen-openapiv2:
+          - 0.0.1
         googleapis_common_protos_version:
-          - 1.53.0
           - 1.62.0
-        grpc_gateway_protoc_gen_openapiv2_version:
-          - 0.1.0
     steps:
       - uses: actions/checkout@v4
       - uses: ./.github/actions/test-dependency-grpc
@@ -92,12 +91,11 @@ jobs:
           - 3.1.3
           - 4.3.3
         protobuf_version:
-          - 3.20.3
+          - 4.25.3
+        protoc-gen-openapiv2:
+          - 0.0.1
         googleapis_common_protos_version:
-          - 1.53.0
           - 1.62.0
-        grpc_gateway_protoc_gen_openapiv2_version:
-          - 0.1.0
     steps:
       - uses: actions/checkout@v4
       - uses: ./.github/actions/test-dependency-grpc