Add translatable aria-label to Youtube button

[audrasjb]

Description

Fixes #7202
This changeset make the Play Youtube video button translated so it's compliant to Web Content Accessibility Guidelines on non-english websites.

Type of change

• New feature (non-breaking change which adds functionality).
• Bug fix (non-breaking change which fixes an issue).
• Enhancement (non-breaking change which improves an existing functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as before).
• Sub-task of #(issue number)
• Chore
• Release

Detailed scenario

See issue #7202

Technical description

Use a gettext function to make the related string translatable.

Mandatory Checklist

Code validation

• I validated all the Acceptance Criteria. If possible, provide screenshots or videos.
• I triggered all changed lines of code at least once without new errors/warnings/notices.
• I implemented built-in tests to cover the new/changed code.

Code style

• I wrote a self-explanatory code about what it does.
• I protected entry points against unexpected inputs.
• I did not introduce unnecessary complexity.
• Output messages (errors, notices, logs) are explicit enough for users to understand the issue and are actionnable.

Additional Checks

• In the case of complex code, I wrote comments to explain it.
• When possible, I prepared ways to observe the implemented system (logs, data, etc.).
• I added error handling logic when using functions that could throw errors (HTTP/API request, filesystem, etc.)

[wordpressfan]

Thanks @audrasjb for your valuable contribution.

This is a dependency code so we need to update the library first here:
https://github.com/wp-media/rocket-lazyload-common/blob/develop/src/Assets.php
Before merging it into WP Rocket because this library is being used in other plugins.

[audrasjb]

Thanks @wordpressfan
Upstream PR added: wp-media/rocket-lazyload-common#122