chore: update 18.04-LTS VHD to 2021.07.16

[jackfrancis]

Reason for Change:

This PR updates the 18.04-LTS VHD to the newest version.

Issue Fixed:

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

If "Yes," did you notify that project's maintainers and provide attribution?

• No
• Yes

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[jackfrancis]

/hold

This won't be ready until the new VHD is actually published.

[mboersma]

/azp run pr-e2e

% az vm image list -f aks -p microsoft-aks -s aks-engine-ubuntu-1804-202007 --all -o table
Offer    Publisher      Sku                            Urn                                                         Version
-------  -------------  -----------------------------  ----------------------------------------------------------  ----------
...
aks      microsoft-aks  aks-engine-ubuntu-1804-202007  microsoft-aks:aks:aks-engine-ubuntu-1804-202007:2021.07.16  2021.07.16

Looks like it's available, so re-running CI.

[azure-pipelines]

No pipelines are associated with this pull request.

[mboersma]

lgtm