Source Code docs: redact bolding from non-UI strings

What does this MR do?

Picks up after !189708 (merged) left off. Cleans up bolding on the remaining Source Code pages, to make life a little easier for the translation team.

I even did myself a solid in !189708 (comment 2476945776) and left myself a list of pages remaining to check!

Pages reviewed in this MR

• doc/user/project/repository/branches/default.md
• doc/user/project/repository/branches/protected.md
• doc/user/project/repository/branches/branch_rules.md
• doc/user/project/repository/push_rules.md
• doc/user/project/repository/files/git_history.md
• doc/administration/settings/account_and_limit_settings.md
• doc/user/project/repository/branches/default.md
• doc/user/project/system_notes.md
• doc/administration/settings/visibility_and_access_controls.md
• doc/user/project/repository/web_editor.md
• doc/user/group/custom_project_templates.md
• doc/user/asciidoc.md
• doc/user/group/ssh_certificates.md
• doc/user/snippets.md
• doc/user/project/repository/signed_commits/gpg.md
• doc/user/project/repository/signed_commits/ssh.md
• doc/user/project/repository/tags/_index.md
• doc/user/project/repository/files/git_blame.md
• doc/user/project/repository/repository_size.md
• doc/user/project/changelogs.md
• doc/integration/sourcegraph.md
• doc/development/project_templates/_index.md
• doc/development/internal_api/_index.md
• doc/development/gitlab_shell/process.md

Related issues

• Related to Source Code docs: remove stray bolding, part 1 (!189708 - merged) which handled the other groupsource code docs
• Closes Create docs: clean up bolding (#537730 - closed) 🎉

Author's checklist

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