Support for Elasticsearch v7

[markus1978]

Elasticsearch 6.8 lost its official support in February 2022. So we should go for Elasticsearch 7, I guess.

• use 7.x in the ci
• use 7.x in the install docs
• added a fix, because 7.x is capping out the total hits instead of actually trying to compute them by default
• fix the index creation. Before, we were trying to add an explicit mapping. But the mapping json was wrong. Therefore, no mapping had been applied effectively and we were running in a dynamic mapping. However, this dynamic mapping did not work with ES 7 anymore due to slight changes in ES. I fixed the mapping so that the ES indices have an explicit non dynamic mapping now.

[codecov]

Codecov Report

Merging #1216 (4fd5aca) into master (1f02600) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 4fd5aca differs from pull request most recent head 3ae2850. Consider uploading reports for the commit 3ae2850 to get more accurate results

@@           Coverage Diff           @@
##           master    #1216   +/-   ##
=======================================
  Coverage   91.27%   91.28%           
=======================================
  Files          72       72           
  Lines        4285     4289    +4     
=======================================
+ Hits         3911     3915    +4     
  Misses        374      374           

Flag	Coverage Δ
project	91.28% <100.00%> (+<0.01%)	⬆️
validator	90.67% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
optimade/server/entry_collections/elasticsearch.py	97.53% <100.00%> (+0.12%)	⬆️

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[markus1978]

@ml-evs I finally fixed all the ES7 issues. This now works for the OPT tests and for our implementation.

[ml-evs]

LGTM, thanks @markus1978!