docs: Remove "beta" mark from message-retention-policy.md.
docs: Remove "beta" mark from message-retention-policy.md.

The retention policy has been sufficiently well tested at this point to
consider it stable.

kou/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,punchagan/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip,rht/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,zulip/zulip,punchagan/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip