Switch to aqtinstall v2 to avoid constantly updating the hard-coded version number of tools

[pzhlkj6612]

We know:

• Qt removes the old version of the tool when a new version is released.
• aqtinstall v1 silently fails when the specified version of a tool does not exist.
• aqtinstall v2 installs tools without specifying the version number.

So we need:

• Switch to aqtinstall v2 in the next major version of this action to use the new install-tool subcommand.
• Tell users (through a ::notice ) that the new version of this action can handle the installation of tools properly.
• Update README and CI script.

Output from aqtinstall v1 when installing the non-existing QIFW 4.0.0:

> aqt tool linux tools_ifw 4.0.0 qt.tools.ifw.40
2021-09-30 01:31:20,378 - aqt - INFO - aqtinstall(aqt) v1.0.0 on Python 3.8.10 [CPython GCC 9.4.0]
2021-09-30 01:31:21,171 - aqt - INFO - Finished installation
2021-09-30 01:31:21,172 - aqt - INFO - Time elapsed: 0.79554108 second

Related: #86 , #101 , #103 , #107 (for README) and #113 (for CI) .