RELEASE_NOTES: Based on the version from 5.1

Name suggested by 7 people on ML

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

RELEASE_NOTES

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+
+            ┌─────────────────────────────────────────┐
+            │ RELEASE NOTES for FFmpeg 7.0 "Dijkstra" │
+            └─────────────────────────────────────────┘
+
+   The FFmpeg Project proudly presents FFmpeg 7.0 "Dijkstra", about 6
+   months after the release of FFmpeg 6.1.
+
+   A complete Changelog is available at the root of the project, and the
+   complete Git history on https://git.ffmpeg.org/gitweb/ffmpeg.git
+
+   We hope you will like this release as much as we enjoyed working on it, and
+   as usual, if you have any questions about it, or any FFmpeg related topic,
+   feel free to join us on the #ffmpeg IRC channel (on irc.libera.chat) or ask
+   on the mailing-lists.