added warnings for deprecated methods

[stevengill]

Summary

This adds a warning if developers are using one of the deprecated methods as described in https://api.slack.com/changelog/2020-01-deprecating-antecedents-to-the-conversations-api

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #992 into master will decrease coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #992      +/-   ##
==========================================
- Coverage   94.55%   94.34%   -0.22%     
==========================================
  Files          12       12              
  Lines         753      760       +7     
  Branches      171      172       +1     
==========================================
+ Hits          712      717       +5     
- Misses         13       14       +1     
- Partials       28       29       +1     

Flag	Coverage Δ
#eventsapi	89.61% <ø> (ø)
#interactivemessages	94.92% <ø> (ø)
#webapi	96.36% <ø> (-0.59%)	⬇️
#webhook	87.50% <ø> (ø)

Impacted Files	Coverage Δ
src/WebClient.ts	95.89% <0.00%> (-0.81%)	⬇️

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