docs: replace dead MyGet build link with CI feed link

[FirmaSpring]

Problem

ISSUE_TEMPLATE.md tells reporters to "Try the MyGet build" and links to https://docs.automapper.io/en/latest/The-MyGet-build.html.

That page is gone:

curl -sL -o /dev/null -w "%{http_code} %{url_effective}\n" \
  https://docs.automapper.io/en/latest/The-MyGet-build.html
# 404 https://docs.automapper.io/en/latest/The-MyGet-build.html

Solution

Link to the CI feed that the project actually publishes to. .github/workflows/ci.yml pushes CI packages to https://f.feedz.io/lucky-penny-software/automapper/nuget/index.json, so the template now points at the feed portal page for that org:

curl -sL -o /dev/null -w "%{http_code} %{url_effective}\n" \
  https://feedz.io/portal/org/lucky-penny-software/repository/automapper
# 200

Verification

Only the markdown link changes; no code or workflow changes.

[CLAassistant]

All committers have signed the CLA.