[SDK-2161] Update release announcement to new sdk-releases channel

[nsingh-branch]

Reference

SDK-2162 -- Create #sdk-releases channel in Slack

Summary

Updated the sync-readme-changelog GHA to send the release announcement slack message in the new #sdk-releases channel instead of #sdk.

Motivation

To make it easier for people to keep track of SDK releases and look back on them.

Type Of Change

• Bug fix (non-breaking change which fixes an issue)

cc @BranchMetrics/saas-sdk-devs for visibility.

[codecov]

Codecov Report

Merging #1316 (9b2e049) into master (97ca6bb) will increase coverage by 3.07%.
Report is 56 commits behind head on master.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1316      +/-   ##
==========================================
+ Coverage   50.57%   53.65%   +3.07%     
==========================================
  Files          68       68              
  Lines       10109    10109              
  Branches     3889     3896       +7     
==========================================
+ Hits         5113     5424     +311     
+ Misses       4748     4432     -316     
- Partials      248      253       +5     

Files	Coverage Δ
BranchSDK/BranchActivityItemProvider.m	0.00% <ø> (ø)
BranchSDK/Branch.m	41.96% <0.00%> (+8.86%)	⬆️

... and 12 files with indirect coverage changes