Migrate neural network-based indicators to new repository (#1701)

* Remove all neural network indicator stuff from `src/`

* Migrate neural network tests

* Migrate neural network examples

* Migrate test dependencies

* Update NEWS.md

* Fix typo

* Remove Requires.jl-based use of Flux.jl

* Fix formatting

* Add migration of indicators to section with breaking changes

---------

Co-authored-by: Hendrik Ranocha <[email protected]>

NEWS.md

@@ -4,6 +4,22 @@ Trixi.jl follows the interpretation of [semantic versioning (semver)](https://ju
 used in the Julia ecosystem. Notable changes will be documented in this file
 for human readability.
 
+## Changes when updating to v0.6 from v0.5.x
+
+#### Added
+
+#### Changed
+
+#### Deprecated
+
+#### Removed
+
+- The neural network-based shock indicators have been migrated to a new repository
+  [TrixiSmartShockFinder.jl](https://github.com/trixi-framework/TrixiSmartShockFinder.jl).
+  To continue using the indicators, you will need to use both Trixi.jl and
+  TrixiSmartShockFinder.jl, as explained in the latter packages' `README.md`.
+
+
 ## Changes in the v0.5 lifecycle
 
 #### Added
@@ -31,6 +47,9 @@ for human readability.
 
 #### Removed
 
+- Migrate neural network-based shock indicators to a new repository
+  [TrixiSmartShockFinder.jl](https://github.com/trixi-framework/TrixiSmartShockFinder.jl).
+
 
 ## Changes when updating to v0.5 from v0.4.x