Remove support for github based devfile registries

[kadel]

Deprecation message was added almost a year ago #4707
v3 is a good time to completely remove this from odo.
Only devfile registries that odo should support is those using official https://github.com/devfile/registry-support

[valaparthvi]

odo init does not seem to support GitHub based registries. This is what I tried.

➜  odo preference registry add RGH2 https://github.com/devfile/registry
 ⚠  Git based registries Deprecated  Please see https://github.com/redhat-developer/odo/tree/main/docs/public/git-registry-deprecation.adoc
New registry successfully added

➜   odo init --name aname --devfile nodejs --devfile-registry RGH2      
 ✗  Downloading devfile "nodejs" from registry "RGH2" [448ms]
 ✗  unable to download devfile: invalid character '<' looking for beginning of value
the command failed, the devfile has been removed from current directory

[kadel]

There are still parts of the code with special logic to handle github registries like

odo/pkg/catalog/catalog.go

Lines 188 to 230 in f298389

	// Github-based registry
	URL, err := convertURL(registry.URL)
	if err != nil {
	return nil, fmt.Errorf("unable to convert URL %s: %w", registry.URL, err)
	}
	registry.URL = URL
	indexLink := registry.URL + indexPath
	request := dfutil.HTTPRequestParams{
	URL: indexLink,
	}
	secure := registryUtil.IsSecure(preferenceClient, registry.Name)
	if secure {
	token, e := keyring.Get(fmt.Sprintf("%s%s", dfutil.CredentialPrefix, registry.Name), registryUtil.RegistryUser)
	if e != nil {
	return nil, fmt.Errorf("unable to get secure registry credential from keyring: %w", e)
	}
	request.Token = token
	}
	jsonBytes, err := dfutil.HTTPGetRequest(request, preferenceClient.GetRegistryCacheTime())
	if err != nil {
	return nil, fmt.Errorf("unable to download the devfile index.json from %s: %w", indexLink, err)
	}
	var devfileIndex []indexSchema.Schema
	err = json.Unmarshal(jsonBytes, &devfileIndex)
	if err != nil {
	if err := util.CleanDefaultHTTPCacheDir(); err != nil {
	log.Warning("Error while cleaning up cache dir.")
	}
	// we try once again
	jsonBytes, err := dfutil.HTTPGetRequest(request, preferenceClient.GetRegistryCacheTime())
	if err != nil {
	return nil, fmt.Errorf("unable to download the devfile index.json from %s: %w", indexLink, err)
	}
	err = json.Unmarshal(jsonBytes, &devfileIndex)
	if err != nil {
	return nil, fmt.Errorf("unable to unmarshal the devfile index.json from %s: %w", indexLink, err)
	}
	}
	return createRegistryDevfiles(registry, devfileIndex)

this should be removed together with tests handling this