Revisions to upgrade instructions for 2.4.0

[jeff-matthews]

Purpose of this pull request

This pull request (PR):

• Removes the "Experimental" label from the Composer upgrade plugin TOC entry. The plugin's GA release is scheduled for 2.4.0 GA.
• Capitalizes "Composer"
• Removes the Upgrade using custom script topic.
• Revises the summary of the script and plugin options.

Affected DevDocs pages

• https://devdocs.magento.com/guides/v2.3/comp-mgr/cli/cli-upgrade.html
• https://devdocs.magento.com/guides/v2.3/comp-mgr/cli/cli-upgrade.html#recreate-magento-updater
• https://devdocs.magento.com/guides/v2.3/comp-mgr/cli/upgrade-with-plugin.html
• https://devdocs.magento.com/guides/v2.3/comp-mgr/cli/upgrade-with-script.html

whatsnew
Removed the topic about using the custom script to upgrade Magento. The Composer update plugin replaces the custom script.

[dshevtsov]

This comment is not directly relevant to TOC update but is more conceptually referred to the comp-mgr/cli/upgrade-with-script.md.

1. The topic is identical in both 2.3 and 2.4. I wonder what was a reason to remove a symlinks (cc @keharper).
2. Do we have a confirmation from @pdohogne-magento that Upgrade using script is relevant for 2.4?

[jeff-matthews]

@pdohogne-magento, can you confirm?

Do we have a confirmation from @pdohogne-magento that Upgrade using script is relevant for 2.4?

[pdohogne-magento]

No, that is no longer relevant for 2.4; it has been fully replaced by the plugin.

[jeff-matthews]

running tests

[ghost]

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