Enabling testing for multiple versions of the Host

[jadahiya-MSFT]

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Description

Summarize the changes, including the goals and reasons for this change. Be sure to call out any technical or behavior changes that reviewers should be aware of.

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Main changes in the PR:

1. Enabling E2E tests against multiple host versions. Currently only enabling it against v4.
2. Added pipeline variable access for the correct branches. $(AppHostLatestBranch) for main and $(AppHostV4ReleaseBranch) for the most recent V4 release.
3. Moved E2E test calls from base azure-pipeline.yaml into a template called web-e2e-versions.yaml