[Bug] ext-protobuf minimum version silently changed in v2.18

What are you really trying to do?

Run CI in temporalio/features.

Describe the bug

Upgrading to v2.18 caused php.Dockerfile to throw the following error:

Unable to unmarshal field `schedule` of class ScheduleDescription
  Unable to unmarshal field `spec` of class Schedule
    Unable to unmarshal field `structured_calendar` of class ScheduleSpec
      Passed value must be a type of array,
      but Google\Protobuf\Internal\RepeatedField given

This is because the Docker image we've been pulling uses ext-protobuf version < 4.0, and v2.18 now requires ext-protobuf >= 4.0. I solved the problem here by upgrading in the Dockerfile. (If there's a better option, let me know!)

Expected resolution: Update the README.md and release notes for 2.18 to note that ext-protobuf < 4.0 is no longer supported.

Minimal Reproduction

You can see the issue here in our CI logs.

Environment/Versions

• SDK version 2.18
• Using Docker

Additional context

Start with README.md and the release notes for version 2.18, then review the linked temporalio/features CI failure and the reported ext-protobuf requirement. Update both documentation locations to state that ext-protobuf versions below 4.0 are unsupported, and verify that the limitation is clearly visible to users upgrading to SDK 2.18.