CXX-3099 perform refname:split=2 within Python

[eramongodb]

Followup to #1384.

due to the available git version (2.43.6)

This was incorrect: RHEL 7.6 distros apparently may contain Git versions as old as 1.8.3 (?!), not just 2.43.6. Although for-each-ref, --format, and %(refname) are supported, :strip=2 is not (requires Git 2.7+):

fatal: unknown refname: format strip=2
Traceback (most recent call last):
  File "etc/calc_release_version.py", line 372, in <module>
    RELEASE_VER = main()
                  ^^^^^^
  File "etc/calc_release_version.py", line 346, in main
    branch_tags = get_branch_tags(active_branch_name)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "etc/calc_release_version.py", line 236, in get_branch_tags
    tags_by_obj = get_object_tags()
                  ^^^^^^^^^^^^^^^^^
  File "etc/calc_release_version.py", line 269, in get_object_tags
    for obj, tagobj, tag in iter_tag_lines():
  File "etc/calc_release_version.py", line 253, in iter_tag_lines
    output = check_output(['git', 'for-each-ref', '--format=%(*objectname)|%(objectname)|%(refname:strip=2)', 'refs/tags/*'])
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "etc/calc_release_version.py", line 131, in check_output
    raise subprocess.CalledProcessError(ret, args[0])
subprocess.CalledProcessError: Command 'git' returned non-zero exit status 128.

Therefore, the strip operation is moved into Python with str.split('/') to drop the refs/tags/ prefix.