Prepare release notes for the release (#1273)

Remove old release notes of already released stuff and add a summary.

Author: llucax

RELEASE_NOTES.md

@@ -2,11 +2,10 @@
 
 ## Summary
 
-<!-- Here goes a general summary of what this release is about -->
+This release provides an experimental, opt-in, time-jumps resilient resampler, that can be enabled by using the new `ResamplerConfig2` class.
 
 ## Upgrading
 
-* The microgrid client dependency has been updated to version 0.9.0
 * The resampling function now takes plain `float`s as values instead of `Quantity` objects.
 * `frequenz.sdk.timeseries.UNIX_EPOCH` was removed, use [`frequenz.core.datetime.UNIX_EPOCH`](https://frequenz-floss.github.io/frequenz-core-python/latest/reference/frequenz/core/datetime/#frequenz.core.datetime.UNIX_EPOCH) instead.