Migrate SLR to Search.g4

[koppor]

In #11803, we migrate(d) away from Lucene syntax (back) to Search.g4.

We need to migrate the web search (and consequently the SLR search).

For SLR, it is OK to define a new version of study.yml, which supports new web search syntax only.

SLR introduced query transformation from Lucene to the publishers at #7350

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Refs: #6805

The converter should be already contained in 1c951fb - for converting the groups to a new syntax.

[koppor]

Can be split into two tasks: First fetcher - and in a follow-up the SLR.

[calixtus]

Dev-Call: split up, focus first on fetcher.

[koppor]

Fetcher done at #13607 🎉

[turhantolgaunal]

/assign-me