Serializing < and > - consistent wording on mutation XSS

[hamishwillee]

#39639 merged a tiny bit prematurely, before I could roll out the agreed wording in #39639 (comment) everywhere. This updates the other places for consistency.

Related docs work can be tracked in #39628

[github-actions]

Preview URLs

• /en-US/docs/Web/API/Element/innerHTML
• /en-US/docs/Web/API/Element/outerHTML
• /en-US/docs/Web/API/ShadowRoot/getHTML
• /en-US/docs/Web/API/ShadowRoot/innerHTML

External URLs (4)

URL: /en-US/docs/Web/API/Element/innerHTML
Title: Element: innerHTML property

• https://research.securitum.com/dompurify-bypass-using-mxss/ (1 time) (Note! This may be a new URL 👀)

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URL: /en-US/docs/Web/API/Element/outerHTML
Title: Element: outerHTML property

• https://research.securitum.com/dompurify-bypass-using-mxss/ (1 time) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/Web/API/ShadowRoot/getHTML
Title: ShadowRoot: getHTML() method

• https://research.securitum.com/dompurify-bypass-using-mxss/ (1 time) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/Web/API/ShadowRoot/innerHTML
Title: ShadowRoot: innerHTML property

• https://research.securitum.com/dompurify-bypass-using-mxss/ (1 time) (Note! This may be a new URL 👀)