Update Nextstrain clade pivots to 23I (BA.2.86)

Based on @trvrb's feedback on Slack¹

> I think the thing to do here is to update clade pivot to 23I and
> update the lineage pivot to JN.1. It's not at all essential that
> they're completely synonymous.

The previous commit already updated the Pango lineage pivot to JN.1,
so this commit updates the Nextstrain clade pivot to 23I (BA.2.86).

¹ https://bedfordlab.slack.com/archives/C036L7V615F/p1708992783913369?thread_ts=1708990294.255439&cid=C036L7V615F

config/config.yaml

@@ -58,14 +58,14 @@ models:
   gisaid:
     nextstrain_clades:
       global:
-        pivot: "23A"
+        pivot: "23I"
     pango_lineages:
       global:
         pivot: "JN.1"
   open:
     nextstrain_clades:
       global:
-        pivot: "23A"
+        pivot: "23I"
     pango_lineages:
       global:
         pivot: "JN.1"

config/mlr-config.yaml

@@ -13,7 +13,7 @@ settings:
 
 model:
   generation_time: 4.8
-  pivot: "23A"
+  pivot: "23I"
   hierarchical: true
 
 inference:

viz/src/App.jsx

@@ -41,8 +41,8 @@ function App() {
         <h2>Clade growth advantage</h2>
         <p>
           These plots show the estimated growth advantage for given clades relative to clade
-          23A (lineage XBB.1.5). This describes how many more secondary infections a variant causes
-          on average relative to clade 23A. Vertical bars show the 95% HPD. The "hierarchical" panel
+          23I (lineage BA.2.86). This describes how many more secondary infections a variant causes
+          on average relative to clade 23I. Vertical bars show the 95% HPD. The "hierarchical" panel
           shows pooled estimate of growth rates across different locations.
           Results last updated {mlrCladesData?.modelData?.get('updated') || 'loading'}.
         </p>