Fix CI build failures caused by Ubuntu 21.10 going EOL

[MishimaHaruna]

Pull Request Prelude

• I have followed proper Hercules code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

This fixes GitHub Actions CI build failures caused by an unsupported Ubuntu distribution not finding its package repositories online.

Ubuntu 21.10, used by the gcc-10, gcc-9 and gcc-8 CI builds is EOL since 2022-07-14. The image has been replaced with alternative versions that include the same packages.

The following changes have been made:

• gcc-10 and gcc-9 have been moved from 21.10 to to 22.04 LTS, which will be under standard support until April 2027 and EOL in April 2032 (workflow gccold1).
• gcc-8 has been moved from 21.10 to to 20.04 LTS, which will be under standard support until April 2025 and EOL in April 2030 (moved from workflow gccold1 to gccold2).
• gcc-7 has been moved from 18.04 LTS to to 20.04 LTS, as described above (workflow gccold2).
• gcc-6, gcc-5 and gcc-4.8 have been left on 18.04 LTS, which will be under standard support until April 2023 and EOL in April 2028 (moved from workflow gccold2 to gccold3).