Update documentation for Linux package installation method name

Problem to solve

Lots of documentation doesn't follow the style guide for installation methods naming: https://docs.gitlab.com/ee/development/documentation/styleguide/index.html#how-to-document-different-installation-methods.

Proposal

Update the documentation in doc directory to align with style guide for Linux package installation methods naming.

Source installation method references can also be updated if they're nearby, otherwise they can be done in a separate effort.

Other links/references

Relates to &10716

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mentioned in merge request !6950 (merged)

Relates to: #8089

mentioned in merge request !7070 (merged)

assigned to @eread

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