Snackbar: Make the explicitDismiss string translatable

[afercia]

Submitting this PR without an associated issue because this is a trivial change that doesn't need a broader discussion.

What?

When snackbars have the explicitDismiss prop set to true, they show an X close button. The button aria-label is not translatable, it's a hardcoded string.

Why?

All user facing text, including non-visible ARIA properties, should be translatable.

How?

Makes the string translatable.

Testing Instructions

N/A

Testing Instructions for Keyboard

Screenshots or screencast

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[afercia]

Introduced in #26952

[Mamaduka]

Thanks for catching the issue, @afercia!

[richtabor]

Noting the copy should likely be "Dismiss notice".

Short, direct commands tend to be easier for users to scan and understand quickly. This phrasing still conveys the essential action without unnecessary words, maintaining clarity and effectiveness.

[afercia]

Thanks @Mamaduka
It seems the PHPunit tests fail very early, it happens on other PRs too for example #60364 so that it seems unrelated. Error message:
docker-compose: command not found lol

Any clue whether it's a known issue?

[Mamaduka]

Sorry, I have no clue regarding the PHP test failure.

[afercia]

Noting the copy should likely be "Dismiss notice".

Unrelated to this PR, anyways a good opportunity to clarify a couple things:

• This kind of common copy / labels etc. should be consistent throughout all the WordPress admin. In core, this copy has been Dismiss this notice since years. I'm not opposed to change it but that should be coordinated across the whole WordPress codebase which is made of Core + Gutenberg. Consistency is key.
• In this specific case, the aria-label is used for an X close button. The button only shows an X graphics. Yes, the aria-label should be meaningful but it should also be understandable based on the graphics in use. For example, speech recognition software users do see an X. They need to understand what the accessible name of this button is, to be able to issue a voice command. An X is commonly used in association with a label that just says 'Close'. I doubt speech recognition software users will ever be able to guess the actual accessible name is 'Dismiss this notice'.

It's important to understand the technologies and devices we have to design for. Anyways, the Snackbar notices would need some important work to be fully accessible that has been discussed at length when they were introduced but no agreement was reached. In my personal opinion, while snaclbars may be a good pattern for small screen devices, they are less than ideal for large screens at the point I would like to see them entirely removed from WordPress.

That said, this PR only aims to fix a non-translatable string. Broader discussions should go in a separate issue.