Remove the term "master" from the docs

[staceysalamon-aiven]

What can we help you with?

Aiven has plans to remove the use of the word "master" in the UI (for example, by using "promote to primary" instead of "promote to master" for PG). In the meantime, to make our documentation more inclusive, we should remove the use of the term "master". Some articles where this term is used:

• https://docs.aiven.io/docs/products/cassandra.html
• https://docs.aiven.io/docs/products/redis/concepts/high-availability-redis.html
• https://docs.aiven.io/docs/products/redis/concepts/memory-usage.html
• https://docs.aiven.io/docs/products/postgresql/concepts/upgrade-failover.html

Of course, we will not be able to change code samples or parameters that require the use of this word.

[lornajane]

Feel free to open followup issues for the code samples, we can share them around people with the expertise in each tech stack?