Reconcile source installation and upgrade docs

What does this MR do?

This MR reconciles differences between the source installation and source upgrade docs.

The documentation on how to install git is copied from the install docs to the major version upgrade docs, so that the latter doesn't contradict the former any longer.

The steps in the major version upgrade docs are moved to be in the same order as in the install docs, so that gems are installed before gitlab-workhorse is installed, which is wrong in the current instance.

The following deprecation warnings from bundler are removed by using the suggested bundle commands

[DEPRECATED] The --deployment flag is deprecated because it relies on being remembered across bundler invocations, which bundler will no longer do in future versions. Instead please use bundle config set deployment 'true', and stop using this flag

[DEPRECATED] The --without flag is deprecated because it relies on being remembered across bundler invocations, which bundler will no longer do in future versions. Instead please use bundle config set without 'development test mysql aws kerberos', and stop using this flag

The last commit changes which local changes are discarded, since this was different between the documentation for patch releases and major upgrades.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content