Session file is broken after vim update

In newer vim version, they changed the script version and some syntax generated by obsession becomes syntax error.

See https://github.com/tpope/vim-obsession/pull/87

To avoid this issue, users have to:

1. Update vim.
2. Update obsession with that fix.
3. Load the session (in old syntax).
4. Save the session (in new syntax).

If a user (me) updated vim without updating obsession:

1. Update vim.
2. Load the session (in old syntax).
3. Save the session (mixed with new syntax and old syntax).

An error will occur next time loading the session.

WDYT to add a check/fix before prosession loading the session file? Then the bug won't affect prosession users even without that fix.

Start by reviewing the session-loading path in prosession and the related fix in vim-obsession pull request #87. Reproduce the failure sequence described in the issue, then determine how session files with old and new Vim syntax should be handled before loading. Done means the mixed-syntax session no longer errors for users who updated Vim first.

Written by the indexing model from the issue text.