Update avian/seasonal flu references

[victorlin]

These references are now outdated following nextstrain/nextstrain.org#811:

• flu/avian
• flu/seasonal

Not urgent, but it'd be good to have updated links at least on doc pages.

Repos to update

• auspice
• augur
• cli
• docs.nextstrain.org

Repos that don't need update

Reasoning inline

• static: archived repo
• sandbox: archived repo
• nextstrain/workflow-asset-summary: only used in a comment
• sphinx-theme: only used as an example
• narratives: only used as an example / tests
• seasonal-flu: used in zoltar/project.json which seems to be unused?

[jameshadfield]

A lot of these uses are in archived repos, some are used intentionally (e.g. nextstrain.org tests), and some don't need changing (e.g. if the usage is the href of a link then it'll redirect). I think the only repos which actually need updates are: auspice, augur, cli, docs.nextstrain.org

[victorlin]

@jameshadfield I moved the checklist into the issue description and also listed the other repos with reasoning on why updates aren't needed.