Update handbook expense limit for monitors

Why is this change being made?

The average price of the monitor of $380 is pretty low, even for a budget monitor. Since we rate monitors as a 10/10 importance for home office workspaces, this is being updated to a more reasonable number of $600 to accommodate the current industry prices for 4K and ultra-wide monitors that many team members buy today.

Since the original budget assumed that most team members used dual monitors and ultra-wide monitors replace 2 monitors, this adds the change that you can expense double the amount for ultra-wide monitors.

4K Prices

• Dell 27" U2720Q $619-779
• Dell 32" U3219Q $864-1,149

Ultra-wide Prices

• Dell 34" P3421W $654-$859 USD - US
• Dell 34" U3421W $929 to $1,159 USD - US
• Dell 38" U3821DW $1,229 to $1,539 USD - US
• Dell 49" U4919DW $1,249 to $1,559 USD - US
• Dell 40" 5K U4021QW $1,679 to $2,099 USD - US
• LG 34" 34WL85C-B $799 USD - US
• LG 38" 38UC99-W $1099 USD - US
• LG 38" 38WN95C-W $1599 USD - US
• LG 49" 49WL95C-WE $1499 USD - US

Examples:

• https://about.gitlab.com/company/culture/all-remote/workspace/#monitors
• https://www.dell.com/en-us/shop/monitors-monitor-accessories/ar/4009?appliedRefinements=2542
• https://dnsmichi.at/2020/04/28/first-time-all-remote-my-new-workspace-feat-standing-desk-and-curved-monitor/

This is an actionable follow up from a brief Slack discussion.
https://gitlab.slack.com/archives/C012ALM8P29/p1614286602043600

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
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • 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. (this requirement has been removed pending identification of a new DRI for the handbook)