[openstreetmap/openstreetmap-website] SoTM Africa 2025 Conference Banner Update (PR #6530)

[Emmanuel Jolaiya]

### Description

Following the recent announcement [on LinkedIn](https://www.linkedin.com/posts/sotmafrica_sotmafrica2025-activity-7394671842260692992-eHFR?utm_source=share&utm_medium=member_desktop&rcm=ACoAACM72y8BIfDB2fDL2SRTc9jzCjgf7AJy0-I) by the SotM Africa organizing team to postpone the conference to June 2026, this PR updates the banner configuration to reflect the new dates and graphics.

Additionally, I cleaned up the past/ongoing State of the Map (SotM) 2025 Europe conference.
You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/6530

-- Commit Summary --

  * chore: cleaup SoTM Europe 2025 conference banner

-- File Changes --

    D app/assets/images/banners/sotm_eu_2025.png (0)
    M config/banners.yml (9)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/6530.patch
https://github.com/openstreetmap/openstreetmap-website/pull/6530.diff

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