Fix typos discovered by codespell

[cclauss]

Fix typos discovered by codespell - https://pypi.org/project/codespell

% codespell

./CHANGELOG.md:24: incompatiblity ==> incompatibility
./elasticsearch_metrics/imps/elastic6.py:198: exsits ==> exists, exist
./elasticsearch_metrics/imps/elastic8.py:583: exsits ==> exists, exist
./elasticsearch_metrics/imps/elastic8.py:772: pass-thru ==> pass-through, pass through, passthrough

% codespell --write-changes

[cclauss]

Resolved git conflicts caused by:

• [ENG-9708] drop elastic6 #99