Allow Spikekill to work on time windows for stddev and variance (#6570)

* Fix: Defect in SpikeKill algorithm was causing a loss of data for Gap Fill and Float methods

* QA: Correct syntax error

* Change MySQL image to MariaDB 10.6 in workflow

This ensures that the page check actually runs as MariaDB and MySQL have both dropped the MD5 function.

* Fix indentation and formatting in spikekill.php

* Fix outlier start value assignment in removespikes.php

* Fix error message for invalid outlier end value

* Fix assignment of out_start and out_end properties

* Add var_kills condition to output array creation

* QA: Align on Floating calculation

* QA: Undefined variable fix

* Feature: Make Spikekill work for Standard Deviation and Variance on Windows

* Changes after Copilot Review

* Another Copolit Recommendation

* More Copilot Reviews

* Fully testing Variance method

* Update lib/spikekill.php

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* One remaining NaN comparision

* More minor issues after Copilot review

* QA: Defaults not calculated correctly

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: TheWitness