Update devcontainer example

[jorickert]

• Updated example to work with current docker image + main branch
• I removed the preinstalled VSCode extensions and settings. IMO suggesting third-party, potential unmaintained extensions is problematic and the settings are often user and IDE specific

[AlexandreEichenberger]

LGTM

[jorickert]

@AlexandreEichenberger the Jenkins s390x runner seems to be stuck

[AlexandreEichenberger]

@jenkins-droid test this please

[AlexandreEichenberger]

Let me restart it, otherwise will ping our CI magician.

[gongsu832]

stderr: fatal: unable to access 'https://github.com/onnx/onnx-mlir.git/': Could not resolve host: github.com

	at PluginClassLoader for git-client//org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandIn(CliGitAPIImpl.java:2849)
	at PluginClassLoader for git-client//org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandWithCredentials(CliGitAPIImpl.java:2189)
	at PluginClassLoader for git-client//org.jenkinsci.plugins.gitclient.CliGitAPIImpl$1.execute(CliGitAPIImpl.java:638)
	at PluginClassLoader for git//hudson.plugins.git.GitSCM.fetchFrom(GitSCM.java:996)
	... 12 more
ERROR: Error fetching remote repo 'origin'
ERROR: Maximum checkout retry attempts reached, aborting

DNS server on s390x Jenkins CI server is dead. Network support has been contacted.