Update re: adding SSH key to the ssh-agent for Mac users (Issue #23159)

[oC-n]

Added a note to step 3 of Adding your SSH key to the ssh-agent for Mac users, for whom the system may continue to ask for SSH passphrase on every git push.

Why:

Following the directions given under Adding your SSH key to the ssh-agent can result in Mac systems continuing to ask for SSH passphrase on every git push. The suggested edit resolves this problem, if imperfectly.

Closes: #23159

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

Adding an extra line to the note for Mac users on Step 3 of Adding your SSH key to the ssh-agent, instructing that ssh-add --apple-use-keychain ~/.ssh/id_ed25519 can be added to .zshrc or .zshrc to resolve the problem of being repeatedly asked for SSH passphrase.

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• 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).
• For content changes, I have completed the self-review checklist.

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[github-actions]

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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

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Source	Preview	Production	What Changed
authentication/connecting-to-github-with-ssh/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent.md	fpt ghec ghes@ 3.9 3.8 3.7 3.6 3.5 3.4 ghae	fpt ghec ghes@ 3.9 3.8 3.7 3.6 3.5 3.4 ghae

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

[cmwilson21]

@oC-n Thanks so much for opening a PR! I'll get this triaged for review ⚡

[am-stead]

Thanks for submitting this PR!✨
I've made a small suggestion so that it aligns with our style guide!

[am-stead]

I've applied the stylistic edit and I have approved the PR for merging ✨

[github-actions]

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