Issue #3618187: Remove the stale CHANGELOG.md (3.x backport)

Fixes #3618187.

Removes CHANGELOG.md. Nothing in the project referenced it, its dated release list stopped at 19-JUN-2025 2.0.0-beta3 so the whole 3.x series was missing, the "Status" section described the unmaintained 8.x-1.x branch, and the alpha/beta/release-candidate checklists recorded work finished years ago. Release notes on the release nodes are where this belongs for a Drupal project.

One thing must be carried across before this is tagged. Under "3.1.0 (#3588828)" the file held the upgrade note for splitting domain_config and domain_config_ui into language-free modules plus domain_config_language and domain_config_language_ui. The architecture is already documented on the documentation site in three languages, but one line is not recorded anywhere else:

Custom code typehinting the moved classes, DomainLanguageConfigFactoryOverride and its interface, DomainLanguageConfigOverride, DomainConfigLanguageManager and DomainLanguageConfigCollectionNameTrait, needs its use statements changed from the Drupal\domain_config namespace to Drupal\domain_config_language.

That is an upgrade instruction rather than a description of the current design, so it belongs in the 3.1.0 final release notes. The split shipped in 3.1.0-alpha4, so it affects anyone coming from 3.0.x and nobody moving between 3.1.0 alphas.

Verified that no file in either branch still mentions the changelog.

Backport of !441 (merged).