Remove use of enable_gcc8 script to fix focal builds

[azeey]

🦟 Bug fix

Summary

Focal based docker containers were failing to build documentation because running enable_gcc8.sh script fails.

Try using:

docker build -t gz-fortress-docs -f Dockerfile.fortress --build-arg GZ_VERSION_PASSWORD=foo --build-arg GZ_VERSION_DATE=5 --build-arg AWS_ACCESS_KEY_ID=foo --build-arg AWS_SECRET_ACCESS_KEY=foo .

and you'd get

STEP 12/28: RUN scripts/enable_gcc8.sh

sudo apt-get install g++-8
Reading package lists...
Building dependency tree...
Reading state information...
The following additional packages will be installed:
  cpp-8 gcc-8 gcc-8-base libgcc-8-dev libmpx2 libstdc++-8-dev
Suggested packages:
  gcc-8-locales g++-8-multilib gcc-8-doc gcc-8-multilib libstdc++-8-doc
The following NEW packages will be installed:
  cpp-8 g++-8 gcc-8 gcc-8-base libgcc-8-dev libmpx2 libstdc++-8-dev
0 upgraded, 7 newly installed, 0 to remove and 0 not upgraded.
Need to get 32.8 MB of archives.
After this operation, 115 MB of additional disk space will be used.
Do you want to continue? [Y/n] Abort.
Error: building at STEP "RUN scripts/enable_gcc8.sh": while running runtime: exit status 1

It's possible to fix enable_gcc8.sh itself to avoid this error, but that script is actually not needed on Focal since gcc-9 is the default there.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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