Manufacturer is the term most often used in this context (meaning the business that actually employs the developers who code the software)
Microsoft (abbreviated as MS) is an American multinational technology
company headquartered in Redmond, Washington, that develops,
manufactures, licenses, supports and sells computer software, consumer
electronics and personal.
--Wikipedia
Also here for another example of the term in use:
Since its foundation in 2001 community4you AG has established itself
as a successful global software manufacturer for the marketing,
development and implementation of future-proof enterprise software.
--Community4uAG
Sometimes it is not the manufacturer of the software that provides support, but the vendor (the person selling it) , in that case if you wanted to distinguish this case you could use vendor, despite not thinking this is a usual term here, it is actually frequently used in the context you specified, providing the support is provided from the company that supplied the software. With large software companies, they distribute through vendors and often not directly, e.g.:
How to Choose the Right Software Vendor
... 5. Check for hidden or additional fees. "Look for hidden costs in the contract, such as additional fees for in-person training, document management services, setup or annual maintenance fees in addition to the monthly support costs,"
So you could use:
"Older operating systems, such as Windows XP and MacOSX 10.8, have
several security weaknesses because they are no longer supported by
their manufacturers"
or
"Older operating systems, such as Windows XP and MacOSX 10.8, have
several security weaknesses because they are no longer supported by
their vendors"
Depending on who is supporting the software.