Handbook: Update Slack channel to social_media_action

Why is this change being made?

Seen the channel rename on Slack and remembered some other handbook pages referencing the Slack channel.

Analysis:

egrep -r '#social_media[\s+`]' sites/handbook

Fix:

find sites/handbook -type f -exec sh -c "gsed -i 's/\#social_media\`/\#social_media_action\`/g' {}" \;

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
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  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.