[Backport] style: change b to strong (a11y)

[DanielRuf]

Original Pull Request

#19421

Description (*)

This PR changes b tags to strong tags which are compliant and the right tag when we want to make something "bold" as this is also relevant for the accessibility.

The <strong>/<b> and <em>/<i> tags have exactly the same effect in most web browsers, but there is a fundamental difference between them: <strong> and <em> have a semantic meaning whereas <b> and <i> only convey styling information like CSS.

While <b> can have simply no effect on a some devices with limited display or when a screen reader software is used by a blind person, <strong> will:

• Display the text bold in normal browsers
• Speak with lower tone when using a screen reader such as Jaws

Fixed Issues (if relevant)

1. magento/magento2#<issue_number>: Issue title
2. ...

Manual testing scenarios (*)

1. ...
2. ...

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @DanielRuf. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me 2.2-develop instance - deploy vanilla Magento instance

For more details, please, review the Magento Contributor Assistant documentation

[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-3650 has been created to process this Pull Request

[magento-engcom-team]

Hi @DanielRuf. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.