Update avro to 1.11.3 (backport to 2.18 branch)

[rafalh]

Update Apache avro library to 1.11.3.
To make it compatible namespace generation for nested classes had to be changed.
Now dots are used instead of dollar signs as indicator of nesting.
AFAIK dollar sign is not an allowed character in Avro namespace according to the specification and this is why this change was implemented in avro library.
Note that avro library generates namespaces not ending with dollar character since version 1.9, but it actually removes all dollar signs in classes with multiple levels of nesting since 1.11.

See: AVRO-2143 and AVRO-2757
Fixes #167

Compatibility:

• over-the-wire protocol should be compatible
• reading schemas from older versions should work
• schemas generated after this change for classes with no nesting are unchanged
• schemas generated after this change for classes with one nesting level are readable by Jackson 2.11+ (commit c570549)
• schemas generated after this change for classes with multiple nesting levels will not be readable by older Jackson versions

[cowtowncoder]

Quick note: as per my comments, happy to review, help get merged.
But one thing we'll need before merging, CLA, from:

https://github.com/FasterXML/jackson/blob/master/contributor-agreement.pdf

It is only needed once before the first contribution, and the usual way is to print it, fill & sign, scan/photo, email to cla at fasterxml dot com.
Once I get that I can proceed with final review and merging.
Thank you in advance, looking forward to FINALLY resolving the upgrade problem!

[rafalh]

I've sent CLA photo. I just noticed that I used info e-mail instead of cla, because that's what was used in CONTRIBUTING.md. Thanks for letting me know.

[cowtowncoder]

@rafalh info is fine too; will need to change CONTRIBUTING.md

[cowtowncoder]

CLA received, will do one more review.

[cowtowncoder]

Thank you again, @rafalh -- this is awesome! Hoping to wrap up 2.18.0-rc1 planning Very Soon Now, to get the release candidate out.