Drop OBJ_VERSION_NO_HEADER_AAD (pack object format v1) support before rc1

Title: Drop OBJ_VERSION_NO_HEADER_AAD (pack object format v1) support before rc1

Body:
`RepoObj.parse()` and `parse_meta()` accept two pack object format versions right now: `OBJ_VERSION_NO_HEADER_AAD` (0x01) and `OBJ_VERSION_HEADER_AAD` (0x02). We only write 0x02; 0x01 is kept so objects from older borg2 betas still read back fine.

We don't owe compatibility across beta versions, so this fallback has no real reason to exist once we're past the current round of betas. Leaving it in after that is just dead weight: an extra branch and an extra accepted version number for nobody.

Before rc1: drop `OBJ_VERSION_NO_HEADER_AAD` from `SUPPORTED_OBJ_VERSIONS` in `src/borg/repoobj.py`, remove the version-0x01 branches in `parse()`/`parse_meta()`, and remove `test_version1_object_without_header_aad_still_readable` in `src/borg/testsuite/repoobj_test.py`.

Reference: #9946