feat(Uno.Sdk.Updater): Add CLI exclude-file support and improve manifest merge logic (backport #1752)

[mergify]

GitHub Issue (If applicable): related to unoplatform/uno#21372

PR Type

What kind of change does this PR introduce?

• Feature
• Improvement

What is the current behavior?

The Uno.Sdk.Updater tool and CI workflow:

• Update all package groups unconditionally.
• Merge manifests in a way that can cause unwanted downgrades.
• Preserve local VersionOverride entries inconsistently, sometimes removing them.

What is the new behavior?

• Added support for excluding package groups from automatic updates using a JSON configuration file (.github/sdk-update-exclude.json) passed via a new CLI argument (--exclude-file).
• Adjusted merge logic to prevent downgrades of local versions.
• Ensured local VersionOverride entries are always preserved and take precedence.
• Added clear logging to confirm when exclusions and overrides are applied.

This gives finer control over dependency updates, avoids regressions, and simplifies local testing.

PR Checklist

Please check if your PR fulfills the following requirements:

• Associated with an issue (GitHub or internal)

Other information

The default exclude file is located at .github/sdk-update-exclude.json.
For local testing, the CLI can be run as:

dotnet run -c Release --project tools/Uno.Sdk.Updater -- --exclude-file .github/sdk-update-exclude.json

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This is an automatic backport of pull request #1752 done by [Mergify](https://mergify.com) + manual adjustment for 6.2 branch