Change admonition title style/syntax

[colleenmcginnis]

The new AsciiDoc serverless docs have lots of admonitions with titles. Admonition titles are currently h3s. I'd like to change them to:

• Not use a heading tag. Using an h3 messes with the "On this page" list of headings and can result in headings that are not necessarily incremental (admonitions can come after h3s, h4s, etc).
• Style using bold text instead of larger text. This is subjective. I just think the current text is too big.

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[lcawl]

I agree it looks better