Use new CircleCI "convenience" images

[paulmelnikow]

Here's a shot at addressing #7725.

After looking through this list, it looks like they provide aliases for the minor node versions but not the major versions.

Closes #7725

[shields-ci]

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[paulmelnikow]

Looks like this is working 👍🏼

[chris48s]

Cheers for having a look at this.
I think the notification we got is actually about the package tests. As I understand it, machine: true requires an update and will be affected by the brownouts:

shields/.circleci/config.yml

Lines 237 to 238 in 7c23f04

	package:
	machine: true

It uses a base image because it then uses nvm to install node versions on top of it.

I think independently of this, the circleci/ images were deprecated last year, so also need updating to the cimg/ ones (so that is still useful) but I think its actually a separate issue.

Its a bit of a pain that there isn't a plain old node:16 or node:17 tag we can use on those

[paulmelnikow]

Aha! Good shout. Will have another pass at that.

[chris48s]

This should be good now