Updates screen iframes need a title attribute

[afercia]

When updating plugins or themes, the updates screen uses some iframes to display the update progress and for accessibility ​iframes need a title attribute to describe their contents.

[afercia]

The proposed patch adds a title attribute to the relevant iframes. For now, it uses Updating... because it's an already existing translatable string. Wondering if should be something more descriptive or if it's better to keep it simple. Any thoughts more than welcome.

Side note: the iframe height="170px" attribute value shouldn't have a unit measure.

[afercia]

To clarify, see the screenshot below: the colored background area is one of the iframes to take care of.

[afercia]

Quickly discussed with the accessibility team and agreed to use the string Update progress.

[afercia]

In 35854:

Accessibility: the iframes used for the update progress in the Updates screen need title attributes to describe their content.

Fixes #34763.

[walbo]

The title attributes isn't translatable.

[afercia]

Ah! My bad :) Thanks @walbo.

[afercia]

In 35855:

Updates screen: make the new strings introduced in [35854] translatable.

Props walbo.
Fixes #34763.