Fix Jenkins migration: Using steps to execute task

[ataillefer]

Why:

To fix the "Using steps to execute tasks" section that uses the wrong Jenkins keyword, IMO.

What's being changed (if available, include any code snippets, screenshots, or gifs):

In the "Using steps to execute task" section of the "Migrate from Jenkins" page, the script entry is replaced by steps, which sounds more relevant in this case. Indeed, script is one particular type of step whereas the paragraph is about groups of steps, handled in Jenkins by the steps element of a declarative Pipeline.

Check off the following:

• [x] I have reviewed my changes in staging (look for the "Automatically generated comment" and click the links in the "Preview" column to view your latest changes).
• [x] For content changes, I have completed the self-review checklist.

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
actions/migrating-to-github-actions/migrating-from-jenkins-to-github-actions.md	fpt ghec ghes@ 3.7 3.6 3.5 3.4 3.3 ghae	fpt ghec ghes@ 3.7 3.6 3.5 3.4 3.3 ghae

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fpt: Free, Pro, Team ghec: GitHub Enterprise Cloud ghes: GitHub Enterprise Server ghae: GitHub AE

[cmwilson21]

@ataillefer Thanks so much for opening a PR! I'll get this triaged for review :zap:

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