Prepare for WTO 1.15 release

[dkwon17]

What does this PR do?

Updates the changelog, CSV, dockerfiles for the 1.15 release.

What issues does this PR fix or reference?

Is it tested? How?

From the root of this repo, set the appropriate environment variables to point to your quay repos and install WTO built from this PR:

# Setup Environment Variables
export WTO_IMG=quay.io/<user>/wto-controller:1.15 && \
export INDEX_IMG=quay.io/<user>/wto-index:1.15 && \
export BUNDLE_IMG=quay.io/<user>/wto-bundle:1.15

# Build custom index 
make build_custom_iib_image

# Register custom catalogsource 
make register_catalogsource 

WTO 1.15 should be viewable on the Installed Operators page of the OpenShift console using the images built from this PR.

[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all