helm: Pin Hubble v0.5.1 in v0.5 branch

[gandro]

Reviewer note: This is PR targeting the v0.5 branch.

In the Cilium documentation, we ask users to obtain the Hubble v0.5
chart via the v0.5 branch. Therefore we should make sure that our chart
refers to v0.5 image tags.

Ref: cilium/cilium#11121

[gandro]

We seem to have a unit test flake in the 0.5 branch:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x100f40b]

goroutine 14 [running]:
github.com/cilium/hubble/pkg/parser/endpoint.ParseEndpointFromModel(0x0, 0x1)
	/home/runner/work/hubble/go/src/github.com/cilium/hubble/pkg/parser/endpoint/endpoint.go:34 +0x4b
github.com/cilium/hubble/pkg/cilium.(*State).consumeEndpointEvents(0xc0003f2180)
	/home/runner/work/hubble/go/src/github.com/cilium/hubble/pkg/cilium/endpoint.go:90 +0x7e9
created by github.com/cilium/hubble/pkg/cilium.TestObserverServer_EndpointAddEvent
	/home/runner/work/hubble/go/src/github.com/cilium/hubble/pkg/cilium/endpoint_test.go:222 +0x990
FAIL	github.com/cilium/hubble/pkg/cilium	0.505s