Remove deprecated `--experimental` flag references from the docs

[andrewmbenton]

Version

1.20.0

What happened?

In particular on this page: https://docs.sqlc.dev/en/stable/guides/using-go-and-pgx.html, but there may be others.

Relevant log output

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Database schema

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SQL queries

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Configuration

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Playground URL

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What operating system are you using?

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What database engines are you using?

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What type of code are you generating?

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