Improve the readability of the getOrder method

[tianshuang]

Ordered.LOWEST_PRECEDENCE + 1 is equal to Ordered.HIGHEST_PRECEDENCE, using Ordered.HIGHEST_PRECEDENCE is more readable.

What is the purpose of the change

Improve the readability of the getOrder method.

Brief changelog

Improve the readability of the getOrder method.

Verifying this change

Same semantics, avoiding integer overflow.

Checklist

• Make sure there is a GitHub_issue field for the change (usually before you start working on it). Trivial changes like typos do not require a GitHub issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
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• Check if is necessary to patch to Dubbo 3 if you are work on Dubbo 2.7
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Add some description to dubbo-website project if you are requesting to add a feature.
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[BurningCN]

LGTM