`SpiLoader` was broken from 1.8.0 to 1.8.1

[JasonMing]

According #1383, SpiLoader was moved from util to spi, this action breaks the user codes which contains direct accessing to SpiLoader.
And, this merging happened during a patch version upgrade? Has no one considered compatibility issues?

[cdfive]

Sorry for the inconvenience. One consideration is that users usually may not use SpiLoader directly. Since the package has been changed, it makes breaking change. If user codes contains direct accessing to SpiLoader, need to change the code if upgrade new version.

[cdfive]

Besides, Sentinel 1.8.1 requires JDK 1.8 or higher version, please see release notes

[wuwen5]

请保持第三位版本号的兼容，sentinel有多个模块，导致这些模块之间不兼容，1.8.1与1.8.0不兼容，建议移动或删除的public类先定义为过期。
例如 使用spring-cloud-starter-alibaba-sentinel-2021.1的场景，依赖了sentinel-xx多个模块，而因为bugfix，用户升级了sentinel-core-1.8.1 模块，导致与sentinel-*** 其他子模块不兼容, 这种不兼容是sentinel自身模块之间的不兼容，没有统一的bom，所以用户得升级配置所有的sentinel子模块.

[JasonMing]

@wuwen5 我们现在做法是只能自己在私库发bom，搞个定时任务每天扫描相关组件生成对应的bom，用户直接import bom来定义版本。
maven的依赖仲裁基于nearest first规则，解释起来模型乱七八糟，没bom真的受不了。。。