fix: add go.mod file to jvm scaffolding dirs (#2414)

Without these the Go toolchain tries to scan the directories and fails
due to the | characters, which are not supported by Go's build system.

Fixes the following failure:

```
info: go: finding module for package github.com/TBD54566975/ftl/go-runtime/ftl
go: downloading github.com/TBD54566975/ftl v0.345.0
go: ftl/builtin imports
	github.com/TBD54566975/ftl/go-runtime/ftl: create zip: jvm-runtime/java/scaffolding/{{ .Name | camel | lower }}/ftl.toml: malformed file path "jvm-runtime/java/scaffolding/{{ .Name | camel | lower }}/ftl.toml": invalid char '|'
jvm-runtime/java/scaffolding/{{ .Name | camel | lower }}/pom.xml.tmpl: malformed file path "jvm-runtime/java/scaffolding/{{ .Name | camel | lower }}/pom.xml.tmpl": invalid char '|'
jvm-runtime/java/scaffolding/{{ .Name | camel | lower }}/src/main/java/{{ .PackageDir }}/EchoVerb.java: malformed file path "jvm-runtime/java/scaffolding/{{ .Name | camel | lower }}/src/main/java/{{ .PackageDir }}/EchoVerb.java": invalid char '|'
jvm-runtime/kotlin/scaffolding/{{ .Name | camel | lower }}/ftl.toml: malformed file path "jvm-runtime/kotlin/scaffolding/{{ .Name | camel | lower }}/ftl.toml": invalid char '|'
jvm-runtime/kotlin/scaffolding/{{ .Name | camel | lower }}/pom.xml.tmpl: malformed file path "jvm-runtime/kotlin/scaffolding/{{ .Name | camel | lower }}/pom.xml.tmpl": invalid char '|'
jvm-runtime/kotlin/scaffolding/{{ .Name | camel | lower }}/src/main/kotlin/{{ .PackageDir }}/EchoVerb.kt: malformed file path "jvm-runtime/kotlin/scaffolding/{{ .Name | camel | lower }}/src/main/kotlin/{{ .PackageDir }}/EchoVerb.kt": invalid char '|'
```

There is an underlying bug that is triggering this, whereby replace
directives aren't correctly propagated to the shared Go FTL module
stubs. Filed #2415 for this.

jvm-runtime/java/scaffolding/go.mod

@@ -0,0 +1,3 @@
+module ignore
+
+go 1.22.6

jvm-runtime/kotlin/scaffolding/go.mod

@@ -0,0 +1,3 @@
+module ignore
+
+go 1.22.6