docs: Update NRQL alert condition migration example references in legacy alert condition documents.

[vagrawal-newrelic]

Description

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Updated the references of migration examples from legacy alert conditions to NRQL alert conditions in synthetic and infra alert condition documents. Also updated headings in migration guide to make it more meaningful. Jira ticket: https://new-relic.atlassian.net/browse/NR-285380

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Type of change

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• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Checklist:

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• [x] My commit message follows conventional commits
• [ ] My code is formatted to Go standards
• [x] I have performed a self-review of my own code
• [x] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes. Go here for instructions on running tests locally.

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