Correct anchor links in Migration Guide

[MikeMcC399]

• This PR is a partial fix of Multiple broken anchors #5630, addressing a few bad anchor links in the Migration Guide caused by minor changes in auto-generated bookmarks or simple typos.

Corrected sections

• Migrating to Cypress 12.0 > Test Isolation
• Migrating to Cypress 10.0 > Config Option Changes > pluginsFile
• Migrating to Cypress 7.0 > Deprecated cy.route2() command removed
• Migrating to Cypress 7.0 > Component Testing

Refactor

Anchor links referring to bookmarks on the same page /guides/references/migration-guide in the section Migrating to Cypress 7.0 > Component Testing are simplified, e.g.
/guides/references/migration-guide#1-Remove-experimentalComponentTesting-config
is simplified to
#1-Remove-experimentalComponentTesting-config

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[MikeMcC399]

Preview

In Migrating to Cypress 12.0
https://deploy-preview-5652--benevolent-cat-040f48.netlify.app/guides/references/migration-guide#Test-Isolation
Scroll down to link discouraged

In Migrating to Cypress 10.0
https://deploy-preview-5652--benevolent-cat-040f48.netlify.app/guides/references/migration-guide#pluginsFile

In Migrating to Cypress 7.0
https://deploy-preview-5652--benevolent-cat-040f48.netlify.app/guides/references/migration-guide#Deprecated-cyroute2-command-removed
Scroll down to "Read more about available functions ..."

https://deploy-preview-5652--benevolent-cat-040f48.netlify.app/guides/references/migration-guide#Component-Testing
Each of the items in the table, "The migration guide contains the following steps:"