Update Roslyn package version in support doc

[JoeRobich]

A new Roslyn 4.3.x package was released and the old package was unlisted. See https://www.nuget.org/packages/Microsoft.CodeAnalysis.Common/4.3.1

[opbld30]

Learn Build status updates of commit 1b5579d:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/extensibility/roslyn-version-support.md	✅Succeeded

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Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

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[jborsecnik]

@leslierichardson95,

Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, add a #sign-off comment to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[opbld33]

Learn Build status updates of commit 1b5579d:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/extensibility/roslyn-version-support.md	✅Succeeded

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[prmerger-automator]

@JoeRobich : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[leslierichardson95]

#sign-off