Delete unused flag 'Retries' from Terraform config

[t-kikuc]

What this PR does:

• Delete 'Retries' from TerraformSyncStageOptions and TerraformApplyStageOptions
• Delete 'Retries' from all docs

Why we need it:

'Retries' has never been used.
It's a trap for users.

cf.
v0.42.0 does not use 'Retries'
https://github.com/pipe-cd/pipecd/blob/v0.42.0/pkg/config/application_terraform.go#L58-L74

Which issue(s) this PR fixes:

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[Warashi]

👍🏻

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.28%. Comparing base (0c99936) to head (554e9e3).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6022      +/-   ##
==========================================
- Coverage   28.28%   28.28%   -0.01%     
==========================================
  Files         523      523              
  Lines       56135    56135              
==========================================
- Hits        15879    15875       -4     
- Misses      39005    39008       +3     
- Partials     1251     1252       +1     

Flag	Coverage Δ
.	23.27% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	65.94% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.33% <ø> (ø)
.-pkg-app-pipedv1-plugin-scriptrun	66.66% <ø> (ø)
.-pkg-app-pipedv1-plugin-terraform	80.26% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-plugin-sdk	50.50% <ø> (-0.21%)	⬇️
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.30% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[t-kikuc]

Let me rerun the lint/go.

🐶 Installing reviewdog ... https://github.com/reviewdog/reviewdog
Error: Error: socket hang up

[ffjlabo]

LGTM