Convert Sentinel-2 MSI sensor name to lowercase in the reader YAML config file and add support for "counts" calibration

[yukaribbba]

This PR has two purposes:

1. The current sensor name in YAML is "MSI", which triggers a minor warning Inconsistent sensor/satellite input in pyspectral since the latter one stores sensor name in lowercase. So it's replace by lowercase.
2. Add "counts" calibration just like other sensors.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.93%. Comparing base (24109a1) to head (4faccbb). Report is 20 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2779 +/- ## ======================================= Coverage 95.92% 95.93% ======================================= Files 377 377 Lines 53535 53541 +6 ======================================= + Hits 51356 51362 +6 Misses 2179 2179 ``` | [Flag](https://app.codecov.io/gh/pytroll/satpy/pull/2779/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [behaviourtests](https://app.codecov.io/gh/pytroll/satpy/pull/2779/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `4.10% <0.00%> (-0.01%)` | :arrow_down: | | [unittests](https://app.codecov.io/gh/pytroll/satpy/pull/2779/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `96.02% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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[coveralls]

Pull Request Test Coverage Report for Build 8721657075

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 8626996485:	0.0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls