chore(changelog): move old changelogs to archive

[liamdebeasi]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
  • Some docs updates need to be made in the ionic-docs repo, in a separate PR. See the contributing guide for details.
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

The changelog file as grown quite large and takes a long time to load. Additionally, GitHub is no longer able to show the raw output because the file is too big.

What is the new behavior?

• Moves the changelogs for Ionic v4 and v5 to the CHANGELOG_ARCHIVE directory. The current major release will use CHANGELOG.md while old major releases will move to this archive so they are still accessible.

Does this introduce a breaking change?

• Yes
• No

Other information

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