Fix ruby_abi_version symbol error on macOS

[SpringMT]

Problem

When building the gem on macOS, the following linker error occurs:

Undefined symbols for architecture arm64:
  "_ruby_abi_version", referenced from:
      <initial-undefines>
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Solution

This PR replaces the static exports.txt with dynamic export file selection based on Ruby version, following grpc's approach.

Changes:

• Remove exports.txt
• Add ext-export.txt (for release versions)
• Add ext-export-with-ruby-abi-version.txt (for development versions)
• Update extconf.rb to dynamically select the appropriate export file based on Ruby version

How it works:

• For Ruby release versions (e.g., 3.2.4): Uses ext-export.txt which only exports Init_zstdruby. Ruby itself provides ruby_abi_version.
• For Ruby development versions (e.g., 3.3.0-dev): Uses ext-export-with-ruby-abi-version.txt as needed.

Related Issues:

• Bug #19289
• grpc/grpc#31970
• redis-client#82
• Darwin: Fix build on development versions of ruby #117