[Meta] Condense nested namespaces

[mjcarroll]

Since we are using C++17, we can replace:

namespace gz {
namespace fuel_tools {

with

namespace gz::fuel_tools

This isn't a high priority change, as it is mostly cosmetic, but it would be nice to get on all of the harmonic branches and then ported forward to ionic.

For each repo, we should:

• Collapse nested namespaces
• Check that include guards are correct.
• Remove using namespace from headers
• (optionally) Remove using namespace from core library sources

I'm using this issue to track progress:

• [x] gz-cmake3
• [ ] gz-common5
• [x] gz-fuel-tools9 https://github.com/gazebosim/gz-fuel-tools/pull/396
• [ ] gz-sim8
• [x] gz-gui8 https://github.com/gazebosim/gz-gui/pull/590
• [ ] gz-launch7
• [ ] gz-math7
• [ ] gz-msgs10
• [ ] gz-physics7
• [ ] gz-plugin2
• [ ] gz-rendering8
• [ ] gz-sensors8
• [ ] gz-tools2
• [ ] gz-transport13
• [ ] gz-utils2
• [ ] sdformat14