Remove unencrypted git protocol use (mininet#1100)

Because Github removes support for unencrypted git protocol in March 2022, URLs in scripts are changed to use https.

See https://github.blog/2021-09-01-improving-git-protocol-security-github/ for details

2.3.0 (mininet#1043)

- 2.3.0rc2 -> 2.3.0
- update copyright date in LICENSE

2.3.0rc2 (mininet#1042)

Update INSTALL

2.3.0b2 (mininet#1022)

+ text file updates

Mininet 2.3.0b1 (mininet#1019)

Also tweak README.md, INSTALL, and CONTRIBUTORS

pass code check