Update dependency letter_opener_web to v2

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
letter_opener_web	'~> 1.4', '>= 1.4.1' -> '~> 2.0'

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Release Notes

fgrehm/letter_opener_web (letter_opener_web)

v2.0.0

Compare Source

• Require Rails >= 5.2, run tests against Rails 6.1 #​113
  • Inline CSS and Javascript, to avoid dependency on asset pipeline #​113
  • Upgrade to Bootstrap 5.1.1 #​113
  • Add rexml gem into dependency for Ruby 3.0 #​106
  • Add routes for Rails API mode #​69
  • Prevent name conflict with Letter class #​108
  • Add Rails' built-in CSRF protection #​111
  • Add Rails' CSP nonce to the script tag #​112
  • Update dev dependencies #​113
  • Switched to using GitHub actions as CI for the project #​113

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