Bump TKR conversion webhook Dockerfile to golang:1.18

[imikushin]

Fix a leftover from Go 1.18 version bump. Follow-up to #3577

What this PR does / why we need it

The TKR conversion webhook Dockerfile must have been hiding a bit too well.

Which issue(s) this PR fixes

N/A

Describe testing done for PR

Release note

Additional information

Special notes for your reviewer

[danniel1205]

lgtm, thanks for the change

[codecov]

Codecov Report

Merging #3640 (4112f51) into main (f173e65) will decrease coverage by 0.83%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3640      +/-   ##
==========================================
- Coverage   46.26%   45.42%   -0.84%     
==========================================
  Files         400      425      +25     
  Lines       39583    41147    +1564     
==========================================
+ Hits        18313    18691     +378     
- Misses      19578    20753    +1175     
- Partials     1692     1703      +11     

Impacted Files	Coverage Δ
cmd/cli/plugin/cluster/get_machinehealthcheck.go	11.42% <0.00%> (ø)
cmd/cli/plugin/cluster/available_upgrade.go	16.32% <0.00%> (ø)
cmd/cli/plugin/cluster/upgrade.go	57.55% <0.00%> (ø)
cmd/cli/plugin/cluster/set_node_pool.go	14.63% <0.00%> (ø)
cmd/cli/plugin/cluster/osimage_oracle.go	3.22% <0.00%> (ø)
cmd/cli/plugin/cluster/create.go	41.30% <0.00%> (ø)
cmd/cli/plugin/cluster/machinehealthcheck.go	100.00% <0.00%> (ø)
...in/cluster/get_machinehealthcheck_control_plane.go	11.11% <0.00%> (ø)
cmd/cli/plugin/cluster/osimage.go	100.00% <0.00%> (ø)
cmd/cli/plugin/cluster/credentials_update.go	9.23% <0.00%> (ø)
... and 17 more

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