Fix deprecation warnings when using bootstrap 4.4.1

Summary

When using bootstrap 4.4.1, assets:precompile gives the following warnings. This will become error in bootstrap 5. So it is a good idea to fix them now.

WARNING: `invisible()` has been deprecated as of v4.3.0. It will be removed entirely in v5.
         on line 8:5 of node_modules/bootstrap/scss/mixins/_deprecate.scss, in mixin `deprecate`
         from line 7:12 of node_modules/bootstrap/scss/mixins/_visibility.scss, in mixin `invisible`
         from line 297:18 of app/assets/stylesheets/pages/issues.scss
         from line 25:9 of app/assets/stylesheets/application.scss

WARNING: `invisible()` has been deprecated as of v4.3.0. It will be removed entirely in v5.
         on line 8:5 of node_modules/bootstrap/scss/mixins/_deprecate.scss, in mixin `deprecate`
         from line 7:12 of node_modules/bootstrap/scss/mixins/_visibility.scss, in mixin `invisible`
         from line 302:16 of app/assets/stylesheets/pages/issues.scss
         from line 25:9 of app/assets/stylesheets/application.scss

Steps to reproduce

Update bootstrap in package.json (yarn upgrade bootstrap@4.4.1).

Example Project

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What is the current bug behavior?

Deprecation warnings, which will soon become bugs in newer versions of bootstrap.

What is the expected correct behavior?

Deprecated methods should be removed in favor or recommended methods.

Relevant logs and/or screenshots

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Output of checks

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Results of GitLab environment info

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`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

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(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

Update the marked scss files to use the recommended APIs instead of deprecated APIs.