fix: correct loggly syslog emergency severity

[moying2026]

Description

This PR corrects the syslog emergency severity name used by the loggly plugin and the RFC5424 utility from EMEGR to EMERG.

EMERG is the standard syslog severity name for severity value 0. The current typo makes severity: "EMERG" fail schema validation and the docs currently advertise the misspelled value.

Checklist

• Correct loggly severity enum from EMEGR to EMERG
• Correct apisix.utils.rfc5424 severity map from EMEGR to EMERG
• Update English and Chinese loggly plugin docs
• Add a regression case to ensure EMERG is accepted and the old misspelling is rejected

Tests

• git diff --check
• make test TEST_FILE=t/plugin/loggly.t (not run successfully in my local environment because OpenResty is not installed: WARNING: OpenResty not found)

[moying2026]

补充一下背景：这个 PR 修正了 loggly 插件和 rfc5424 工具里 syslog emergency 级别的拼写错误（EMEGR → EMERG），同时更新了中英文文档，并补了一个回归测试。本地因为没装 OpenResty，没能跑完整套测试，但改动范围很小，只涉及字符串修正和文档，不影响其他逻辑。麻烦有空时帮忙 review，谢谢。