Prepare for release of v3.1

[mattdot]

This pull request includes updates to the changelog and the removal of redundant unreleased change files. The key changes involve adding new features and fixing existing issues in the terraform-provider-power-platform project.

Changelog Updates:

• .changes/3.1.0.md: Added entries for version 3.1.0, including new features and fixes.
• CHANGELOG.md: Updated to include the new version 3.1.0 with added features and fixes.

Removal of Redundant Files:

• .changes/unreleased/added-20240930-125638.yaml: Removed redundant unreleased change file for unique_name addition.
• .changes/unreleased/added-20241002-142330.yaml: Removed redundant unreleased change file for disable_on_destroy attribute.
• .changes/unreleased/fixed-20240927-111725.yaml: Removed redundant unreleased change file for environment_type update trigger.
• .changes/unreleased/fixed-20241001-211351.yaml: Removed redundant unreleased change file for powerplatform_rest resources fix.

Configuration Adjustment:

• .changie.yaml: Adjusted the changeFormat to use a specific GitHub repository URL for issues.