Update perlmutter timeline for 2023-12-20 maint
This MR updates the perlmutter timeline with changes for the 2023-12-20 maintenance.
Submitters and reviewers please make sure
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internal links are relative and to the .md file -
avoid links named "here" -
alt-text for images is descriptive -
doesn't duplicate existing content -
spell check -
renders correctly -
does not include any identifiable information (real usernames, project names, etc)