[release-1.21] pilot: fix bug in endpoint updates

[istio-testing]

Manual cherrypick required.

#50746 failed to apply on top of branch "release-1.21":

Applying: pilot: fix bug in endpoint updates
Using index info to reconstruct a base tree...
M	pilot/pkg/model/endpointshards.go
M	pilot/pkg/serviceregistry/serviceregistry_test.go
Falling back to patching base and 3-way merge...
Auto-merging pilot/pkg/serviceregistry/serviceregistry_test.go
Auto-merging pilot/pkg/model/endpointshards.go
Applying: Prod impl
Using index info to reconstruct a base tree...
M	pilot/pkg/model/endpointshards.go
M	pilot/pkg/model/service.go
M	pilot/pkg/serviceregistry/serviceregistry_test.go
Falling back to patching base and 3-way merge...
Auto-merging pilot/pkg/serviceregistry/serviceregistry_test.go
Auto-merging pilot/pkg/model/service.go
Auto-merging pilot/pkg/model/endpointshards.go
CONFLICT (content): Merge conflict in 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 0002 Prod impl
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".

[tanujd11]

Closing the issue as the 1.21 cherry-pick was merged.