release docs for v3.4.0

[mawasile]

This pull request updates the changelog for the upcoming 3.4.0 release and removes outdated unreleased change files. The most important changes include new features, changes, fixes, and documentation improvements.

New Features:

• Added Managed identity and Workload Identity Federation authentication (.changes/3.4.0.md, CHANGELOG.md). [1] [2]
• Added explicit error for powerplatform_environment when provisioning a new environment in a selected Azure region (.changes/3.4.0.md, CHANGELOG.md). [1] [2]
• Added new powerplatform_copilot_studio_application_insights resource (.changes/3.4.0.md, CHANGELOG.md). [1] [2]

Changes:

• powerplatform_environment attributes environment_group_id and billing_policy_id now use an empty GUID 00000000-0000-0000-0000-000000000000 as no value instead of an empty string (.changes/3.4.0.md, CHANGELOG.md). [1] [2]

Fixes:

• Added warning for powerplatform_data_record when something other than set is used for columns dynamic attribute (.changes/3.4.0.md, CHANGELOG.md). [1] [2]
• Fixed issues with powerplatform_environment_group_rule_set throwing RuleSetInvalid error (.changes/3.4.0.md, CHANGELOG.md). [1] [2]
• Fixed bug where updating display_name field in powerplatform_environment triggers manager environment to enable itself (.changes/3.4.0.md, CHANGELOG.md). [1] [2]

Documentation:

• Improved documentation for powerplatform_environment_application_package_install resource (.changes/3.4.0.md, CHANGELOG.md). [1] [2]
• Corrected telemetry_optout documentation regarding the User-Agent header (.changes/3.4.0.md, CHANGELOG.md). [1] [2]