Fix PR reference in CHANGELOG.md 1.51.0

[tnir]

Ruby 2.6 runtime support was actually dropped in PR: #11791, not in #10791.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote [good commit messages][1].
• [n/a] Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• [n/a] Squashed related commits together.
• [n/a] Added tests.
• [n/a] Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [n/a] Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

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• Fixes up Drop Ruby 2.6 runtime support #11791

[bbatsov]

Thanks!