Fix registry name for test apps images (kanisterio#832)

## Change Overview

Switch to ghcr registry for test app images for upcoming releases

## Pull request type

Please check the type of change your PR introduces:
- [ ] 🚧 Work in Progress
- [x] 🌈 Refactoring (no functional changes, no api changes)
- [ ] 🐹 Trivial/Minor
- [ ] 🐛 Bugfix
- [ ] 🌻 Feature
- [ ] 🗺️ Documentation
- [ ] 🤖 Test



## Test Plan

Manually validated tests by running `make integration-test`
```
OK: 5 passed
--- PASS: Test (301.68s)
PASS
ok      github.com/kanisterio/kanister/pkg/testing      301.706s
~/go-ws/src/github.com/kanisterio/kanister
```

- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

pkg/app/cassandra.go

@@ -56,6 +56,7 @@ func NewCassandraInstance(name string) App {
 			Chart:    "cassandra",
 			RepoName: helm.BitnamiRepoName,
 			Values: map[string]string{
+				"image.registry":       "ghcr.io",
 				"image.repository":     "kanisterio/cassandra",
 				"image.tag":            "0.44.0",
 				"image.pullPolicy":     "Always",

pkg/app/mongodb.go

@@ -61,6 +61,7 @@ func NewMongoDB(name string) App {
 			Chart:    "mongodb",
 			Values: map[string]string{
 				"architecture":     "replicaset",
+				"image.registry":   "ghcr.io",
 				"image.repository": "kanisterio/mongodb",
 				"image.tag":        "0.44.0",
 			},

pkg/app/postgresql.go

@@ -50,6 +50,7 @@ func NewPostgresDB(name string, subPath string) App {
 			RepoURL:  helm.BitnamiRepoURL,
 			Chart:    "postgresql",
 			Values: map[string]string{
+				"image.registry":                        "ghcr.io",
 				"image.repository":                      "kanisterio/postgresql",
 				"image.tag":                             "0.44.0",
 				"postgresqlPassword":                    "test@54321",