Fix CI: Redmine master doesn't support for Ruby v2,7

[otegami]

Issue

CI failed because of the following error.

+ bundle install
Your Ruby version is 2.7.8, but your Gemfile specified >= 3.0.0, < 3.4.0

ref: https://github.com/otegami/redmine_wiki_extensions/actions/runs/8227000508/job/22494241147

Cause

Redmine master(verison 6?) doesn't support for Ruby 2.7

• ref: https://github.com/redmine/redmine/blob/c3fe22476231adff5f6fef2e1692be8024dd0c44/Gemfile#L3

Solutions

We can just exclude the Ruby version 2.7 matrix with Redmine master.

[codeclimate[bot]]

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