fix(db-backup): age pre-migration dumps out with the nightly ones

Pre-migration dumps were kept by count alone, so an install that updates
rarely kept its last two, and every account deleted since, indefinitely,
while the docs and the backups card stated the nightly retention as the
retention of deleted accounts. A pre-migration dump older than the oldest
nightly dump kept now goes, except the newest one, the way back from the
latest update, which stays until the next update. The card, the disaster
recovery guide and its variable table say so.

src/components/admin/backups/__tests__/database-dump-row.test.tsx

@@ -49,7 +49,8 @@ describe("<DatabaseDumpRow>", () => {
   it("names the newest good dump, where it is, and how long dumps keep deleted accounts", () => {
     const html = render(HEALTHY);
     expect(html).toContain("kept on this server and off-host");
-    expect(html).toContain("the newest 7 are kept on this server");
+    expect(html).toContain("the newest 7 nightly dumps are kept on this server");
+    expect(html).toContain("before the latest update stays until the next update");
     expect(html).toContain("removed after 30 days");
     expect(html).not.toContain('data-slot="database-dump-failed"');
     expect(html).not.toContain('data-slot="database-dump-setup"');