Remove executable name option from help

[maxwell-k]

This is only one approach to addressing #1150 and may not be preferred.

• I have added an entry to docs/changelog.md

Summary of changes

This is a change to the help text for install and run to remove the description of passing an executable name. After this change the help does not suggest invocations including --python python3.11 which lead to #1150 . Fixes #1150

Test plan

Tested by running:

.nox/develop-3-12/bin/python src/pipx install --help
.nox/develop-3-12/bin/python src/pipx run --help

Example excerpt from output:

  --python PYTHON       Python to run with. Possible values can be the version to pass to py launcher
                        (3.11) or the full path to the executable. Requires Python 3.8 or above.

[maxwell-k]

After some testing without python-launcher installed I am not sure this is the right fix.