Update for Spring Cloud Azure 6.4.0/7.3.0 released

[Netyyyy]

Description

Update for Spring Cloud Azure 6.4.0/7.3.0 released

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[Copilot]

Pull request overview

Updates Spring Cloud Azure reference metadata and documentation to reflect the newly released Spring Cloud Azure 6.4.0 (Spring Boot 3.5.x line) and 7.3.0 (Spring Boot 4.0.x line), including the latest compatibility upper bound for Spring Boot 4.0.6.

Changes:

• Bumped Spring Cloud Azure artifact versions and GitHub/sample/javadoc links from 6.3.0/7.2.0 to 6.4.0/7.3.0 in the Spring reference YAML, and extended compatibility ranges to 4.0.6.
• Updated the Spring Boot/Spring Cloud ↔ Spring Cloud Azure/azure-sdk-bom mapping table to reflect 6.4.0/7.3.0 and azure-sdk-bom 1.3.7.
• Added timeline entries for the 6.4.0 and 7.3.0 releases.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
sdk/spring/spring-reference.yml	Updates Spring Cloud Azure versions, compatibility ranges, and related links used by the Spring reference metadata.
docs/spring/Spring-Versions-Mapping.md	Updates the documented version mapping table for Spring Boot/Spring Cloud to Spring Cloud Azure and azure-sdk-bom.
docs/spring/Spring-Cloud-Azure-Timeline.md	Adds timeline entries documenting the 6.4.0 and 7.3.0 releases and their dependency baselines.