ruby: make bigdecimal a development dependency

[mwear]

Fixes #28927

I ran into this while packaging the OpenTelemetry Ruby auto-instrumentation as a prebuilt gem bundle (see #28927 for the full write up). The tl;dr: bigdecimal is only used by the test suite, so it only needs to be a development dependency. The only references are:

protobuf/ruby/tests/common_tests.rb

Line 9 in 8baed75

require 'bigdecimal'

protobuf/ruby/tests/common_tests.rb

Line 1708 in 8baed75

m.duration = BigDecimal("5")

As a runtime dependency it causes pain for anyone vendoring the gem. bigdecimal is a native extension with no precompiled build, so it has to be compiled from source for each Ruby ABI, making it the one gem that stops a prebuilt bundle from working across Ruby versions, despite never being loaded at runtime.

This PR moves bigdecimal from a runtime to a development dependency. For consistency it also moves bigdecimal into the test group in the Gemfile, since that's the only environment that needs it.

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