Make private Pattern field transient

[melissalinkert]

See glencoesoftware/bioformats2raw#139 (comment)

Using Java 17 without this PR:

$ cat test.pattern
test_c<1-3>.fake
$ showinf -cache test.pattern
...
failed to save memo file: /Users/melissa/test-case/.test.pattern.bfmemo
...

The same test with this PR should successfully save the memo file.

Technically breaks memo files (to fix them!), so probably not OK for a patch release.

[sbesson]

A run of the automated tests against our curated cellomics datasets with JDK17 indicates all tests are passing including the memo file generation/reloading.

It looks unlikely that Bio-Formats will be successfully tested on JDK 17 across all file formats by the time we are ready to release Bio-Formats 6.10.0. As we are going for a minor release and this PR invalidate cache files, it feels safe enough to include as a first step.