[Minor] Remove 'posting in the mailing list' in contributing.md

[Shivshankar-Reddy]

With referece of below slack discussion: I have created this PR to remove the statemnet about mailing list and updated the file.

Mateusz Warzynski
Yesterday at 7:44 AM
Hello everyone! It's nice to meet you.
I've discussed possible observability enhancements in Valkey with
@Irfan Ahmad
, which lead to drafting the design proposal. I believe the process of contribution is outlined here.
Start by posting in the mailing list and creating an issue at GitHub with the description of, exactly, what you want to accomplish and why.
I've published a draft proposal for observability enhancements in Valkey: #1167. Does this message satisfy the 'posting in the mailing list'?

Viktor Söderqvist (Ericsson)
Yesterday at 8:00 AM
'posting in the mailing list'
We've never had a mailing list. 😂 This must be left from redis times. We need to delete this sentence.
Posting an issue is a great first step. Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.63%. Comparing base (b0f23df) to head (aba8585).
Report is 1 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1174      +/-   ##
============================================
- Coverage     70.64%   70.63%   -0.01%     
============================================
  Files           114      114              
  Lines         61740    61740              
============================================
- Hits          43615    43609       -6     
- Misses        18125    18131       +6     

see 13 files with indirect coverage changes

[zuiderkwast]

Thanks!