Use existing public 2y builds as “representative” trees in our reports

[huddlej]

The nextflu-private builds include all strains with available titer data which makes them less useful for metrics like LBI or looking at regional frequencies. We make separate representative builds for each lineage, but we should just use the public builds since those builds use representative sampling already. To use these public builds in our reports, we need to make the following changes:

• Remove representative builds from the nextflu-private build configuration
• Replace references to those representative builds in the narrative Markdowns with references to builds on the date of each report @2023-11-17
• Update the nextstrain-public build config to use the same subclade colors for all lineages that we use in the private reports, so we display consistent colors throughout reports across datasets. This step will require separate Auspice config JSONs for HA and NA for the 2y builds and will probably require splitting out the 2y builds into their own blocks of the build config YAML.