Add Note about dropping go 1.18

[MadVikingGod]

This is a simple change to note the dropping support of 1.18

[codecov]

Codecov Report

Merging #3771 (9e49e97) into main (b177f58) will increase coverage by 0.0%.
The diff coverage is n/a.

❗ Current head 9e49e97 differs from pull request most recent head dd10a1b. Consider uploading reports for the commit dd10a1b to get more accurate results

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3771   +/-   ##
=====================================
  Coverage   81.7%   81.7%           
=====================================
  Files        167     167           
  Lines      12453   12459    +6     
=====================================
+ Hits       10175   10181    +6     
  Misses      2063    2063           
  Partials     215     215           

Impacted Files	Coverage Δ
exporters/jaeger/jaeger.go	90.3% <0.0%> (-0.9%)	⬇️
sdk/metric/pipeline.go	90.5% <0.0%> (-0.1%)	⬇️
sdk/metric/meter.go	89.5% <0.0%> (ø)
sdk/metric/instrument.go	95.7% <0.0%> (ø)
sdk/metric/metricdata/data.go	57.1% <0.0%> (ø)
metric/instrument/syncint64.go	100.0% <0.0%> (ø)
metric/instrument/asyncint64.go	100.0% <0.0%> (ø)
metric/instrument/instrument.go	100.0% <0.0%> (ø)
exporters/prometheus/exporter.go	83.5% <0.0%> (ø)
metric/instrument/syncfloat64.go	100.0% <0.0%> (ø)
... and 4 more