Use standard SPDX identifier for license

[indeyets]

Various automatic tools do exact checks against strings listed here https://spdx.org/licenses/

cluster-key-slot uses upper-case name which isn't properly matched

[CLAassistant]

All committers have signed the CLA.

[indeyets]

Examples of such tools (list not exhaustive):

• https://github.com/davglass/license-checker
• https://github.com/HansHammel/license-compatibility-checker

[coveralls]

Pull Request Test Coverage Report for Build 33

• 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 100.0%

---

Totals
Change from base Build 31:	0.0%
Covered Lines:	50
Relevant Lines:	50

---

💛 - Coveralls

[Salakar]

Thank you, I will get this published tomorrow

[Uzlopak]

@Salakar

Can you please publish it?
Our pipeline did not handle the license identifier wll ;)

fastify/fastify-redis#158