The parameter Commitish in sparse-checkout.yml is misspelled

Commitish should have 2 't's when not hyphenated.

https://github.com/Azure/azure-sdk-tools/blob/52a573351e9a5e1ae0476349b2880b77e66d819e/eng/common/pipelines/templates/steps/sparse-checkout.yml#L7-L10

https://git-scm.com/docs/gitglossary#Documentation/gitglossary.txt-aiddefcommit-ishacommit-ishalsocommittish
> **commit-ish (also committish)**
> A commit object or an object that can ...

We should allow for the correct spelling "committish" and the current spelling "commitish" for compatibility (possibly with a deprecation warning).

Start by reading eng/common/pipelines/templates/steps/sparse-checkout.yml around lines 7-10 and compare the parameter spelling with the linked Git glossary. Check how the parameter is consumed in the template, then verify that both “committish” and the existing “commitish” spelling remain supported, with a deprecation warning if the project’s conventions provide one.

Written by the indexing model from the issue text.