Now that Drupal 8.8 is no longer supported, remove the security coverage constants

Problem/Motivation

Drupal 9.1.0 has been released, and with it, core 8.8.x has lost security support. It makes no sense to keep the security coverage constants that have now expired in the current code, as there won't be any backports to 8.8.x anymore.

Proposed resolution

Remove the respective lines in core/modules/update/src/ProjectSecurityData.php

Remaining tasks

• Remove those lines.
• Discuss whether the code handling SECURITY_COVERAGE_ENDING_WARN_DATE_x_y constants is still useful.

User interface changes

None

API changes

Two constants are removed from ProjectSecurityData.php

Data model changes

N/A

Release notes snippet

N/A