dmg-canvas: use versioned url

[samford]

Important: Do not tick a checkbox if you haven’t performed its action. Honesty is indispensable for a smooth review process.

In the following questions <cask> is the token of the cask you're submitting.

After making any changes to a cask, existing or new, verify:

• The submission is for a stable version or documented exception.
• brew audit --cask --online <cask> is error-free.
• brew style --fix <cask> reports no offenses.

Additionally, if adding a new cask:

• Named the cask according to the token reference.
• Checked the cask was not already refused.
• Checked the cask is submitted to the correct repo.
• brew audit --cask --new <cask> worked successfully.
• HOMEBREW_NO_INSTALL_FROM_API=1 brew install --cask <cask> worked successfully.
• brew uninstall --cask <cask> worked successfully.

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In reviewing dmg-canvas recently, I noticed that versions can be found on an upstream page listing previous versions, which includes the current one. Assuming this is updated in a timely fashion (i.e., when a new release is made), we can use the versioned zip files and check the aforementioned page for version information instead of having to use ExtractPlist.

Related to #171006.