[ISSUE #15171] Remove startup bash re-exec logic

[jay666mnj]

What is the purpose of the change

Remove the bash re-exec logic introduced by #15177 now that startup.sh has been made sh-compatible by #15178.

This PR also updates startup-native.sh to use POSIX-compatible conditionals, so it no longer needs the bash re-exec guard.

Brief changelog

• Remove bash re-exec guard from startup.sh
• Remove bash re-exec guard from startup-native.sh
• Replace remaining bash-only [[ ... ]] conditionals in startup-native.sh with POSIX-compatible [ ... ]

Verifying this change

• Select-String -Path distribution\bin\startup.sh,distribution\bin\startup-native.sh -Pattern "BASH_VERSION|exec bash|Re-execute|re-exec"
• rg -n "\[\[|\]\]|&>|read -p" distribution\bin\startup.sh distribution\bin\startup-native.sh
• git diff --check

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• Write necessary unit-test to verify your logic correction.
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[github-actions]

Thanks for your this PR. 🙏
Please check again for your PR changes whether contains any usage/api/configuration change such as Add new API , Add new configuration, Change default value of configuration.
If so, please add or update documents(markdown type) in docs/next/ for repository nacos-group/nacos-group.github.io

---

感谢您提交的PR。 🙏
请再次查看您的PR内容，确认是否包含任何使用方式/API/配置参数的变更，如：新增API、新增配置参数、修改默认配置等操作。
如果是，请确保在提交之前，在仓库nacos-group/nacos-group.github.io中的docs/next/目录下添加或更新文档（markdown格式）。

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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