refactor(e2e): migrate to external oadp-must-gather container image

Remove local must-gather directory and build process in favor of using the
external quay.io/konveyor/oadp-must-gather:latest image via oc adm must-gather.
This eliminates architecture mismatch issues and keeps must-gather code in its
dedicated repository.

Changes:
- Updated RunMustGather() in tests/e2e/lib/apps.go to use oc adm must-gather
- Added MUST_GATHER_IMAGE env var (defaults to quay.io/konveyor/oadp-must-gather:latest)
- Removed build-must-gather target from Makefile
- Removed entire must-gather/ directory (3,174 lines deleted)
- Updated documentation in TESTING.md

The SKIP_MUST_GATHER flag is preserved for skipping must-gather collection.
Version-specific images can be used by setting MUST_GATHER_IMAGE env var.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: kaovilai

Makefile

@@ -789,7 +789,7 @@ OADP_BUCKET ?= $(shell cat $(OADP_BUCKET_FILE))
 SETTINGS_TMP=/tmp/test-settings
 
 .PHONY: test-e2e-setup
-test-e2e-setup: login-required build-must-gather
+test-e2e-setup: login-required
 	mkdir -p $(SETTINGS_TMP)
 	TMP_DIR=$(SETTINGS_TMP) \
 	OPENSHIFT_CI="$(OPENSHIFT_CI)" \
@@ -902,10 +902,3 @@ endif
 		$(SED) -i "s%resources:%resources:\n- $$file_name%" $(shell pwd)/config/samples/kustomization.yaml;done
 	@make bundle
 
-.PHONY: build-must-gather
-build-must-gather: check-go ## Build OADP Must-gather binary must-gather/oadp-must-gather
-ifeq ($(SKIP_MUST_GATHER),true)
-	echo "Skipping must-gather build"
-else
-	cd must-gather && go build -mod=mod -a -o oadp-must-gather cmd/main.go
-endif

docs/developer/testing/TESTING.md

@@ -25,7 +25,8 @@ To get started, you need to provide the following **required** environment varia
 | `TEST_HCP` | Exclusively run Hypershift  backup/restore testing | `false` | false |
 | `TEST_UPGRADE` | Exclusively run upgrade tests. Need to first run `make catalog-test-upgrade`, if testing non production operator | `false` | false |
 | `TEST_CLI` | Exclusively run CLI-based backup/restore testing | `false` | false |
-| `SKIP_MUST_GATHER` | must-gather is compiled locally in the Makefile, may cause issue if local and cluster arch do not match| `false` | false |
+| `SKIP_MUST_GATHER` | Skip running must-gather collection during E2E tests | `false` | false |
+| `MUST_GATHER_IMAGE` | Container image to use for must-gather collection via `oc adm must-gather` | `quay.io/konveyor/oadp-must-gather:latest` | false |
 
 > **Note:**