Bump version to 15.5 LTS #3556

Job	Run time
build (production)	3m 22s
build (test)	4m 5s
Libvips tests (3.1)	3m 30s
End to End testing (3.1)	4m 19s
Libvips tests (3.2)	4m 3s
test (3.1)	9m 53s
End to End testing (3.2)	3m 29s
Back to original and return test (.ruby-version)	11m 40s
Libvips tests (.ruby-version)	3m 41s
test (3.2)	7m 55s
End to End testing (.ruby-version)	2m 23s
test (.ruby-version)	9m 5s
Elastic Search integration testing (3.1, docker.elastic.co/elasticsearch/elasticsearch:7.17.13)	3m 35s
Elastic Search integration testing (3.2, docker.elastic.co/elasticsearch/elasticsearch:7.17.13)	3m 51s
Elastic Search integration testing (.ruby-version, docker.elastic.co/elasticsearch/elasticsearch:...	3m 26s
Elastic Search integration testing (.ruby-version, docker.elastic.co/elasticsearch/elasticsearch:...	3m 49s
Elastic Search integration testing (.ruby-version, opensearchproject/opensearch:2)	3m 40s
	1h 25m 46s

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