bump etcd image version to 3.5.15-0

[erhancagirici]

Description of your changes

https://github.com/upbound/spaces/pull/1708 bumps etcd image version used in spaces 1.8
This PR adds the mirrored etcd image version to 3.5.15-0 for spaces >=1.8.x
and keeps the 3.5.6-0 with the compatible version<=1.7.x

Complements #620

I have:

• Read and followed Upbound's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

Build up cli

make build

for spaces 1.8.x

_output/bin/darwin_arm64/up space mirror --dry-run \
                  --version=1.8.0-rc.1 \
                  --to-dir=~/spaces-images-test | grep etcd

# output:
# crane pull xpkg.upbound.io/spaces-artifacts/etcd:3.5.15-0 ~/spaces-images-test/etcd-3.5.15-0.tgz

for spaces <=1.7.x

_output/bin/darwin_arm64/up space mirror --dry-run \
                  --version=1.7.2 \
                  --to-dir=./spaces-images | grep kubectl

# output:
# crane pull xpkg.upbound.io/spaces-artifacts/etcd:3.5.15-0 ~/spaces-images-test/etcd-3.5.6-0.tgz