My Jetpack: useInstallPlugins() hook- Fix translations to account for localization.

[elliottprogrammer]

An Automattic repo member that works on our translations left a comment in one of my previous PR's, asking to update some strings to account for localization.
This PR applies those updates so that the strings can be better translated and account for localization.

Related to: #41013 (review)

Proposed changes:

• Defined the singular and plural version of the string in 2 separate strings, which accounts for localization.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

N/A

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• I think it would be enough to simply inspect the code change and verify that the singular and plural version of the success message are defined separately and that change looks reasonable.

I've manually tested this PR, and if you would like to test it too, do the following:

• Run this PR on a Jurassic Ninja site via the Jetpack Beta Plugin (run this branch on the Jetpack plugin).
• Make sure Jetpack is connected (both site and user).
• Purchase Jetpack Complete.
• In My Jetpack you should see a Notice, suggesting to install Boost & CRM (See screenshot)
• 
• Click the CTA to install both plugins.
• In the top right corner, look for a Toast popout that says, "Plugins installed successfully!" Verify you see this Toast message. See screenshot:
• 
• Now deactivate and uninstall the Boost plugin.
• In My Jetpack you should see the Notice again, but suggesting to install only Boost this time.
• Click the CTA to install Boost.
• In the top right corner, look for a Toast popout that says, "Jetpack Boost installed successfully!" Verify you see this Toast message. See screenshot:
• 

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[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the fix/use-install-plugins-translations branch.

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  bin/jetpack-downloader test jetpack fix/use-install-plugins-translations
  

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[github-actions]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/my-jetpack/_inc/data/products/use-install-plugins.ts	0/12 (0.00%)	0.00%	3 ❤️‍🩹

Full summary · PHP report · JS report

[IanRamosC]

LGTM!