Bump dart from 3.8.3-sdk to 3.9.0-sdk in /.github/docker (#1960)

Bumps dart from 3.8.3-sdk to 3.9.0-sdk.

<details>
<summary>Most Recent Ignore Conditions Applied to This Pull
Request</summary>

| Dependency Name | Ignore Conditions |
| --- | --- |
| dart | [>= 3.0.a, < 3.1] |
| dart | [>= 3.1.a, < 3.2] |
| dart | [>= 3.2.a, < 3.3] |
| dart | [>= 3.3.a, < 3.4] |
| dart | [>= 3.4.a, < 3.5] |
| dart | [>= 3.5.a, < 3.6] |
</details>


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</details>

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/docker/Dockerfile.dart

@@ -1,3 +1,3 @@
-FROM dart:3.8.3-sdk
+FROM dart:3.9.0-sdk
 
 CMD echo this is a dummy file used to automate dependency upgrades for plugins