Add Bengaluru to the GraphQL day announcement post (#2379)

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Closes #<issue number>

## Description

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Author: martinbonnin

src/pages/blog/2026-04-02-graphql-day-2026.mdx

@@ -17,6 +17,7 @@ For 2026, you can expect GraphQL Days in the cities below:
 - **[Singapore](https://graphql.org/day/2026/singapore/)** - April 14–15
 - **[New York City](https://graphql.org/day/2026/nyc/)** - May 13–14
 - **[Amsterdam](https://graphql.org/day/2026/amsterdam/)** - June 9-10 
+- **[Bengaluru](https://graphql.org/day/2026/bbengaluru/)** - August 19-20
 - **[Melbourne](https://graphql.org/day/2026/melbourne/)** - October 28–29
 - **[Paris](https://graphql.org/day/2026/paris/)** - December 1–3