Add GHES 3.15 to GHAS feature matrix

Fixes #44

Add GHES 3.15 to the GHAS feature matrix.

* **Release Notes**
  - Add GHES 3.15 to the Release Notes section with the release date, deprecation date, and release notes link.
* **Secret Scanning**
  - Add GHES 3.15 to the Secret Scanning section with the features introduced in GHES 3.15.
  - Add new features such as "Secret scanning for discussions, issues, and pull request titles, bodies, and comments", "Create a push protection bypass API endpoint", and "Enabling detection of non-provider patterns for an organization".
* **Code Scanning**
  - Add GHES 3.15 to the Code Scanning section with the features introduced in GHES 3.15.
  - Add new features such as "CodeQL can scan C# projects without a build".
* **Supply-chain Security**
  - Add GHES 3.15 to the Supply-chain security section with the features introduced in GHES 3.15.
* **Security Overview**
  - Add GHES 3.15 to the Security Overview section with the features introduced in GHES 3.15.
* **Administration**
  - Add GHES 3.15 to the Administration section with the features introduced in GHES 3.15.
  - Add new feature "CodeQL pull request alerts view".

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For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/advanced-security/advanced-security-material/issues/44?shareId=XXXX-XXXX-XXXX-XXXX).

Author: felickz