Update transitive markdown parser dependency (#12)

Using https://pkg.go.dev/golang.org/x/vuln/cmd/govulncheck to validate [the
CVE] it notes that:

```
Scanning your code and 340 packages across 57 dependent modules for known vulnerabilities...

=== Informational ===

Found 1 vulnerability in packages that you import, but there are no call
stacks leading to the use of this vulnerability. You may not need to
take any action. See https://pkg.go.dev/golang.org/x/vuln/cmd/govulncheck
for details.

Vulnerability #1: GO-2023-2074
    Parser out-of-bounds read vulnerability caused by a malformed markdown input
  More info: https://pkg.go.dev/vuln/GO-2023-2074
  Module: github.com/gomarkdown/markdown
    Found in: github.com/gomarkdown/markdown@v0.0.0-20230716120725-531d2d74bc12
    Fixed in: github.com/gomarkdown/markdown@v0.0.0-20230922105210-14b16010c2ee

No vulnerabilities found.

Share feedback at https://go.dev/s/govulncheck-feedback.
```

This means that for most users of this package, they are unaffected, but
to make sure that we keep this package CVE free, we can update the
transitive dependency.

We cannot update Iris, which pulls in this dependency, due to it now
requiring Go 1.21, and we do not want to require Go 1.21 for consumers.

Co-authored-by: Paul Imbert <9633306-pimbert@users.noreply.gitlab.com>
Co-authored-by: Jamie Tanna <jamie.tanna@elastic.co>

[the CVE]: https://security.snyk.io/vuln/SNYK-GOLANG-GITHUBCOMGOMARKDOWNMARKDOWNPARSER-5916451

go.mod

@@ -32,7 +32,7 @@ require (
 	github.com/go-playground/validator/v10 v10.14.1 // indirect
 	github.com/goccy/go-json v0.10.2 // indirect
 	github.com/golang/snappy v0.0.4 // indirect
-	github.com/gomarkdown/markdown v0.0.0-20230716120725-531d2d74bc12 // indirect
+	github.com/gomarkdown/markdown v0.0.0-20230922112808-5421fefb8386 // indirect
 	github.com/gorilla/css v1.0.0 // indirect
 	github.com/iris-contrib/schema v0.0.6 // indirect
 	github.com/josharian/intern v1.0.0 // indirect

go.sum

@@ -56,8 +56,8 @@ github.com/goccy/go-json v0.10.2/go.mod h1:6MelG93GURQebXPDq3khkgXZkazVtN9CRI+MG
 github.com/golang/protobuf v1.5.0/go.mod h1:FsONVRAS9T7sI+LIUmWTfcYkHO4aIWwzhcaSAoJOfIk=
 github.com/golang/snappy v0.0.4 h1:yAGX7huGHXlcLOEtBnF4w7FQwA26wojNCwOYAEhLjQM=
 github.com/golang/snappy v0.0.4/go.mod h1:/XxbfmMg8lxefKM7IXC3fBNl/7bRcc72aCRzEWrmP2Q=
-github.com/gomarkdown/markdown v0.0.0-20230716120725-531d2d74bc12 h1:uK3X/2mt4tbSGoHvbLBHUny7CKiuwUip3MArtukol4E=
-github.com/gomarkdown/markdown v0.0.0-20230716120725-531d2d74bc12/go.mod h1:JDGcbDT52eL4fju3sZ4TeHGsQwhG9nbDV21aMyhwPoA=
+github.com/gomarkdown/markdown v0.0.0-20230922112808-5421fefb8386 h1:EcQR3gusLHN46TAD+G+EbaaqJArt5vHhNpXAa12PQf4=
+github.com/gomarkdown/markdown v0.0.0-20230922112808-5421fefb8386/go.mod h1:JDGcbDT52eL4fju3sZ4TeHGsQwhG9nbDV21aMyhwPoA=
 github.com/google/go-cmp v0.5.5 h1:Khx7svrCpmxxtHBq5j2mp/xVjsi8hQMfNLvJFAlrGgU=
 github.com/google/go-cmp v0.5.5/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/google/go-querystring v1.1.0 h1:AnCroh3fv4ZBgVIf1Iwtovgjaw/GiKJo8M8yD/fhyJ8=