Use `spec.version` in `gemspec` instead of constant

[sambostock]

Since we've already assigned

spec.version = OpenTelemetry::Instrumentation::<%= pascal_cased_instrumentation_name %>::VERSION

we can use spec.version instead of interpolating the fully qualified constant, which makes things a little more readable.

Why bother?

I generated a new integration but it got the constant name slightly wrong. Because we use the constant name multiple times, I had to manually fix more occurrences than I would have had to with this change. 🤷

[plantfansam]

Hello, and thank you for your contribution!

We recently split Ruby instrumentation out into the opentelemetry-ruby-contrib repo.

This PR is related to instrumentation, so we'll need you to re-open it against opentelemetry-ruby-contrib. Sorry for the inconvenience!

To do that, you can:

1. Create a fork of opentelemetry-ruby-contrib and copy the git url
2. In your opentelemetry-ruby repo, run git remote add tmp-contrib <your-fork-git-url>
3. git push tmp-contrib your-branch-name
4. Open a new PR in contrib (feel free to just copy/paste your original PR description there)
5. Close your open PR in this repo with a comment that links to your new PR in contrib
6. Delete your tmp-contrib remote from opentelemetry-ruby (git remote rm tmp-contrib)
7. git clone your opentelemetry-ruby-contrib fork, check out your branch, and make all changes in that repo from now on!

Sorry again for the inconvenience, and thank you for contributing!

[github-actions[bot]]

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[kaylareopelle]

👋 Hi, @sambostock! Thank you for your PR!

As Sam mentioned, Ruby instrumentation now lives in the opentelemetry-ruby-contrib repo. We'd be happy to take another look at your work in that context.

Since this PR is focused on instrumentation, I'm going to close it.

We appreciate your contribution and hope to work with you again soon!