Support google-protobuf >= 4.26 in gemspec

[qnighy]

Problem

protobuf v26.0 had been released, and it contains breaking changes to the Ruby runtime. This resulted in the gem being bumped from 3.25 to 4.26.

However, as googleapis-common-protos-types and grpc restricts the google-protobuf version within 3.x, the new version is virtually unavailable as of now.

Expected solution

The version restriction is relaxed from "~> 3.18" to ">= 3.18", "< 5.0". Additionally, incompatibilities are fixed if any. Note that the biggest change (to my understanding) is the Ruby DSL removal and it is already resolved in #165.

[dazuma]

Thanks @qnighy! Yes, we will get a relaxed version restriction through the rest of the api clients stack once we've been able to verify compatibility.

[colinbendell]

What is the expected eta of supporting the 4.x series?