Pin Importer plugin version with old PHP support - 5.2

[desrosj]

This fixes the failing WordPress Importer plugin tests by pinning the final version of the plugin with support for PHP < 7.2.

Trac ticket: Core-63983

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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Test this pull request with WordPress Playground.

[desrosj]

@swissspidy I've made the version dynamic so that the primary branch continues to be tested on 7.2+. Could you give it another look over for me?

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props desrosj, swissspidy.

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[desrosj]

Also created #9895 for the 6.5 branch, which is the latest branch with support for PHP < 7.2.

[desrosj]

Merged in r60762.