release notes - Escape shortcodes in changelog files

[cderv]

when importing them as render target in quarto-web

Also, fix the existing ones.

files <- list.files("docs/download/changelog", pattern = "_changelog.md", recursive = TRUE)
for (file in files) {
 xfun::gsub_file(file, pattern = "(?<!\\{)(\\{\\{<[^>]*>\\}\\})(?!\\})", replacement = "{\\1}", perl = TRUE)
}

This fixes quarto-dev/quarto-cli#13322

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[cderv]

This PR helps at least fix then issue in 1.5 changelog

And prevent this specific problem in the future.

It may not be the long term solution as their could be other quarto-only syntax at some point, but if so, then I guess we'll have other problems too (as changelog.md is used in more places)