Update documentation links that link to beta versions of Borg docs.

docs/how-to/customize-warnings-and-errors.md

@@ -48,7 +48,7 @@ support for [more granular exit
 codes](https://borgbackup.readthedocs.io/en/1.4-maint/usage/general.html#return-codes)
 means that you can configure borgmatic to respond to specific Borg conditions.
 See the full list of [Borg 1.4 error and warning exit
-codes](https://borgbackup.readthedocs.io/en/1.4.0b1/internals/frontends.html#message-ids).
+codes](https://borgbackup.readthedocs.io/en/1.4.0b2/internals/frontends.html#message-ids).
 The `rc:` numeric value there tells you the exit code for each.
 
 For instance, this borgmatic configuration elevates all Borg backup file

docs/how-to/make-per-application-backups.md

@@ -133,7 +133,7 @@ For Borg 1.x, use a shell pattern for the `match_archives` value and see the
 [Borg patterns
 documentation](https://borgbackup.readthedocs.io/en/stable/usage/help.html#borg-help-patterns)
 for more information. For Borg 2.x, see the [match archives
-documentation](https://borgbackup.readthedocs.io/en/2.0.0b5/usage/help.html#borg-help-match-archives).
+documentation](https://borgbackup.readthedocs.io/en/2.0.0b8/usage/help.html#borg-help-match-archives).
 
 Some borgmatic command-line actions also have a `--match-archives` flag that
 overrides both the auto-matching behavior and the `match_archives`

docs/how-to/upgrade.md

@@ -151,7 +151,7 @@ borgmatic rcreate --verbosity 1 --encryption repokey-aes-ocb \
 ```
 
 Read about [Borg encryption
-modes](https://borgbackup.readthedocs.io/en/2.0.0b5/usage/rcreate.html#encryption-mode-tldr)
+modes](https://borgbackup.readthedocs.io/en/2.0.0b8/usage/rcreate.html#encryption-mode-tldr)
 for more details.
 
 To transfer data from your original Borg 1 repository to your newly created
@@ -174,7 +174,7 @@ confirmation of success—or tells you if something hasn't been transferred yet.
 Note that by omitting the `--upgrader` flag, you can also do archive transfers
 between related Borg 2 repositories without upgrading, even down to individual
 archives. For more on that functionality, see the [Borg transfer
-documentation](https://borgbackup.readthedocs.io/en/2.0.0b5/usage/transfer.html).
+documentation](https://borgbackup.readthedocs.io/en/2.0.0b8/usage/transfer.html).
 
 That's it! Now you can use your new Borg 2 repository as normal with
 borgmatic. If you've got multiple repositories, repeat the above process for