[release-1.20] Fix service entry merge

[istio-testing]

Manual cherrypick required.

#50711 failed to apply on top of branch "release-1.20":

Applying: Fix service entry merge
Applying: Add test
Applying: add release notes
Applying: better test
Applying: working except stable ordering
Using index info to reconstruct a base tree...
M	pilot/pkg/serviceregistry/serviceentry/conversion.go
M	pilot/pkg/xds/cds_test.go
Falling back to patching base and 3-way merge...
Auto-merging pilot/pkg/xds/cds_test.go
Auto-merging pilot/pkg/serviceregistry/serviceentry/conversion.go
Applying: sort
Applying: Refactor
Using index info to reconstruct a base tree...
M	pilot/pkg/model/endpointshards.go
M	pilot/pkg/model/push_context.go
M	pilot/pkg/model/service.go
M	pilot/pkg/serviceregistry/serviceentry/conversion.go
Falling back to patching base and 3-way merge...
Auto-merging pilot/pkg/serviceregistry/serviceentry/conversion.go
Auto-merging pilot/pkg/model/service.go
Auto-merging pilot/pkg/model/push_context.go
Auto-merging pilot/pkg/model/endpointshards.go
Applying: fix tests
Using index info to reconstruct a base tree...
M	pilot/pkg/model/endpointshards.go
M	pilot/pkg/model/push_context.go
M	pilot/pkg/model/service.go
M	pilot/pkg/serviceregistry/serviceentry/conversion.go
Falling back to patching base and 3-way merge...
Auto-merging pilot/pkg/serviceregistry/serviceentry/conversion.go
Auto-merging pilot/pkg/model/service.go
Auto-merging pilot/pkg/model/push_context.go
CONFLICT (content): Merge conflict in pilot/pkg/model/push_context.go
Auto-merging pilot/pkg/model/endpointshards.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0008 fix tests
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

[zirain]

fixed by #50776