PHP 8.1 update

[alexgit2k]

• Set return-type for some functions
• Removed serializeable

Running without deprecation notices in PHP 8.1, solr-search is also working, but I'm not sure if this fix is really correct.

Fixes #27

[0mars]

@alexgit2k thank you so much, will look into it <3

[0mars]

known side-effects:

=====================================================================
TIME END 2022-03-14 18:48:48

=====================================================================
TEST RESULT SUMMARY
---------------------------------------------------------------------
Exts skipped    :    0
Exts tested     :   33
---------------------------------------------------------------------

Number of tests :  151               151
Tests skipped   :    0 (  0.0%) --------
Tests warned    :    0 (  0.0%) (  0.0%)
Tests failed    :   15 (  9.9%) (  9.9%)
Tests passed    :  136 ( 90.1%) ( 90.1%)
---------------------------------------------------------------------
Time taken      :   10 seconds
=====================================================================

=====================================================================
FAILED TEST SUMMARY
---------------------------------------------------------------------
SolrClient::getDebug() - Get request debug logs for the last request [tests/019.solrclient_getdebug.phpt]
SolrDocument::hasChildDocuments() - Method test [tests/025.solrdocument_haschilddocuments.phpt]
SolrDocument::getChildDocumentsCount() - Method test [tests/026.solrdocument_getchilddocscount.phpt]
SolrDocument::getInputDocument() - Where document has child docs [tests/027.solrdocument_getinputdocument_children.phpt]
SolrDocument - clone [tests/028.solrdocument_clone.phpt]
SolrDocument - clone [tests/029.solrdocument_serialize.phpt]
SolrDocument::clear - Remove all fields from the document [tests/031.solrdocument_clear.phpt]
SolrDocument::fieldExists [tests/032.solrdocument_fieldexists.phpt]
SolrDocument::getField [tests/037.solrdocument_getfield.phpt]
SolrResponse - ParseMode [tests/101.solrresponse_parseMode.phpt]
SolrDocument - Response parsed as SolrDocument with child documents [tests/106.solrresponse_child_doc_response_solrdoc.phpt]
Solr - Fetch and Update nested documents [tests/160.solr_update_document_block.phpt]
SolrParams::serialize() - serialize params [tests/196.solrparams_serialize.phpt]
SolrParams::unserialize() [tests/197.solrparams_unserialize.phpt]
Solr bug #61836 - Extending SolrDocument [tests/bug_61836_error.phpt]

[remicollet]

This PR breaks compatibility with older version...