Issue #3590125: Revert the (>=2.1) info.yml dep constraint — breaks dev-version users

The (>=2.1) constraint added in MR !1 (merged) (released as 1.0.1) silently rejects users running xls_serialization's 2.1.x-dev branch. PHP's version_compare treats the x in 2.1.x-dev as a string less than any digit, so 2.1.x-dev compares as less than 2.1 and the isCompatible() check in SystemRequirements::checkRequirements() returns FALSE.

composer.json already enforces ^2.1 at install time and understands dev-version constraints natively, so the info.yml dep can fall back to a presence check without losing real enforcement.