[SE-43] Add and use version utility, fix 2.8.x button sizes

[dremin]

Summary

Adds a semver-based version-checking utility to the template to make it easier for features to validate supported Flex UI versions.

Refactored existing version-checking code to use this new utility.

Added version checks for task canvas header buttons so that they use the correct size in 2.8.x, which was increased.

Note: The semver package was already included in our plugin bundle due to other dependencies pulling it in; this should not increase plugin size.

Checklist

• [x] Tested changes end to end
• [x] Requested one or more reviewers

[github-actions[bot]]

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