[plugin] Display a warning when compiling on Amazon Linux 2 and optin documentation and examples for Amazon Linux 2023 (#668)

This PR has been reworked. Instead of silently switching the default
base image based on Swift version, we now:

1. **Keep Amazon Linux 2 as the default** base Docker image for the
packager plugin
2. **Add a prominent deprecation warning** when AL2 is used (either via
Docker or natively), informing developers that AL2 reaches End of Life
on June 30, 2026
3. **Migrate all examples** (READMEs, SAM templates, scripts) to build
and deploy on Amazon Linux 2023 (`provided.al2023` runtime +
`--base-docker-image swift:amazonlinux2023`)
4. **Update documentation** (readme, quick-setup) with migration notes

The warning includes the `--base-docker-image swift:6.3-amazonlinux2023`
flag and reminds developers to use the `provided.al2023` runtime when
deploying.

After June 30, 2026, the default will switch to AL2023.

---

<details>
<summary>Original PR description (superseded)</summary>

~~Now that Docker Hub has official Swift images based on Amazon Linux
2023 (starting with 6.3), the packager plugin picks the right base image
automatically depending on the Swift version:~~
~~- Swift 6.3 and later: `swift:<version>-amazonlinux2023`~~
~~- Earlier versions: `swift:<version>-amazonlinux2` (unchanged
behavior)~~
~~- No version specified (latest): defaults to `amazonlinux2023`~~

~~When only a major version is provided (e.g. `--swift-version 6`
without a minor), we conservatively treat it as 6.0 and use Amazon Linux
2, since we can't be sure it's 6.3+.~~
~~Also added a verbose log line showing the resolved Swift version,
Amazon Linux version, and final base image to help with debugging.~~
~~The `--base-docker-image` flag still overrides everything as before.~~

</details>

---------

Co-authored-by: Sébastien Stormacq <stormacq@amazon.lu>

Examples/MultiSourceAPI/README.md

@@ -13,7 +13,7 @@ Based on the successfully decoded type, it returns an appropriate response.
 ## Building
 
 ```bash
-swift package archive --allow-network-connections docker
+swift package archive --allow-network-connections docker --base-docker-image swift:amazonlinux2023
 ```
 
 ## Deploying

Examples/MultiSourceAPI/template.yaml

@@ -8,7 +8,7 @@ Resources:
     Properties:
       CodeUri: .build/plugins/AWSLambdaPackager/outputs/AWSLambdaPackager/MultiSourceAPI/MultiSourceAPI.zip
       Handler: provided
-      Runtime: provided.al2
+      Runtime: provided.al2023
       Architectures:
         - arm64
       MemorySize: 256